Even with High Sierra, the Optiplex780 / 760 is not quite at the end. We're still playing!
the Optiplex story:
10.10: http://www.insanelymac.com/forum/topic/302877-guide-yosemite-1010x-dell-optiplex-780-760/
10.11: http://www.insanelymac.com/forum/topic/308097-guide-el-capitan-1011x-dell-optiplex-780-760-755-790-990/
10.12: http://www.insanelymac.com/forum/topic/312656-guide-macos-sierra-1012-dell-optiplex-780-760-755-790-990/
10.13: this guide
Note: High Sierra supports only CPUs with SSE4 support, example: my Q9400 or my Q8200 working with Sierra. Core2Duo E7200 / Core2Quad Q8200 or higher works as well. See CPU-X.app for your CPU !!!
For a direct update from 10.12 to 10.13, you first install the latest version of Clover.
Copy your Kext's from Clover/kexts/10.12 to /other.
Note new BIOS data and firmware data in Clover's config.plist in SMBIOS. For more information read on.
Instructions for Clean Install:
1. Download Install macOS High Sierra.app
2. CreateInstall USB flash drive (8GB) with terminal (If problems with USB, then create internal Install HDD): sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/USB --applicationpath /Applications/Install\ macOS\ High\ Sierra.app --nointeraction  
3. Install current version Clover for HighSierra-Support to your Install medium (Legacy-Method):
Install Clover in the ESP
Bootloader / Install boot0af in the MBR
CloverEFI / Clover-EFI 64Bit SATA
Drivers64UEFI / EmuVariableUefi-64
Drivers64UEFI / OsxAptioFixDrv-64
Drivers64UEFI / PartitionDxe-64
Install RC Scripts In Target Volume
Theme, free choice
Install Clover Preference Pane
4. Copy your config.plist and DSDT from your Sierra HDD or SSD or new users from here
mount your EFI partition
DSDT.aml insert to EFI/Clover/ACPI/patched/
config.plist insert to EFI/Clover/
Attention! New bios data and new firmware data.
Use current version CloverConfigurator!

Example Optiplex780/760:
my_DSDT.aml.zip (my DSDT, optimized for Core2Quad, configured for HD5450 and Audio LayoutID 13 (0D) for AppleALC + Lilu. Additionally Framebuffer in the Clover is "Shrike" for the HD 5450)
my_config.plist.zip (Create serial number, board serial number, MLB, ROM !!!)

Example Optiplex755:
by @ricoc90 (files and info): http://www.insanelymac.com/forum/topic/332512-guide-macos-high-sierra-1013-dell-optiplex-780-760-755-790-990/#entry2598770
Example Optiplex790/990: by @morpheousman (files and info): http://www.insanelymac.com/forum/topic/332512-guide-macos-high-sierra-1013-dell-optiplex-780-760-755-790-990/#entry2595193 by @chris1111 (files and info): http://www.insanelymac.com/forum/topic/332512-guide-macos-high-sierra-1013-dell-optiplex-780-760-755-790-990/#entry2595284
Thanks @morpheousman and @chris1111
5. Copy your kexts of your Sierra HDD or SSD to EFI/Clover/kexts/other/
these are my kexts for 10.13: other.zip
6. Important preparation with HD5450 (780/760/755)
With High Sierra, there is a black screen with the HD5450, because C2D and C2Q have no Injector iGPU. Since High Sierra, the HD5450 needs a rollback from Sierra.
WhateverGreen.kext + Lilu.kext does not work with LegacyBios, unfortunately.
Save in advance the following Kext's from your Sierra installation from /System/Library/Extensions:
IOAcceleratorFamily2.kext   Note: Optiplex 790 (or higher) with i? CPU with Injector GPU does not require rollback.
7. apfs.efi
This is the current apfs.efi from 10.13.4 DP2:
The current original apfs.efi can be found after each install or update in usr/standalone/i386/ (use ShowAllFiles!).
The new file system is only for SSDs. HDDs continue to work with HFS+.
copy the apfs.efi to EFI/Clover/drivers64/
8. Now High Sierra install on your HDD or SSD (Partitions with Guid scheme)
The installation, even with subsequent updates, can take place in several steps.
My experiences:
APFS two steps
HFS+ one step
8.1 With the HD5450 there is a black screen the first time after the install.
I take the SSD out of the Dell and connect it to another Mac.
Here I am using the app "EasyKext Pro" and doing the rollback from the backup (step 6 above - AMDRadeonX3000.kext, AMDRadeonX3000GLDriver.bundle, IOAccelerator2D.plugin and IOAcceleratorFamily2.kext from Sierra 10.12.6).
Reinstall SSD in the Dell and now we get a picture in High Sierra.
after installation:
9. Terminal (for unsigned apps) sudo spctl --master-disable  
10. Install Clover on your HDD or SSD (Legacy-Method):
Install Clover in the ESP
Bootloader / Install boot0af in the MBR
CloverEFI / Clover-EFI 64Bit SATA
Drivers64UEFI / EmuVariableUefi-64
Drivers64UEFI / OsxAptioFixDrv-64
Drivers64UEFI / PartitionDxe-64
Install RC Scripts In Target Volume
Theme, free choice
Install Clover Preference Pane
11. Copy your config.plist, DSDT, Kext´s and apfs.efi to your High Sierra HDD or SSD
after installation sound:
Please ask if you have problems with the AppleALC method! It is not hard to get it right.
optional Audio Optiplex780/760:
current AppleALC + Lilu in the Clover's Kext folder works for the Optiplex780/760 with:
LayoutID 11 (0B) ManualMode
LayoutID 13 (0D) autodetect Speaker/HP
The current release can be found here:
optional Audio Optiplex755:
trial_AppleALC_Lilu_Optiplex755_layoutID28_1C.zip LayoutID 28 (1C) please report
optional Audio Optiplex790 and 990:
current AppleALC + Lilu in the Clover's Kext folder works for the Optiplex790/990 with:
LayoutID 15 (0F) ManualMode
LayoutID 16 (10) autodetect Speaker/HP
Night Shift Patch:
Replace the original in /System/Library/PrivateFrameworks/ and reboot.
Tested graphics cards with High Sierra:
my XFX one HD5450: with DSDT patch and frame buffer "Shrike" works great with HDMI audio in the Optiplex780 with kext rollback from Sierra
@morpheousman´s HD5450 Bios-Patch: http://www.insanelymac.com/forum/topic/312656-guide-macos-sierra-1012-dell-optiplex-780-760-755-790-990/page-18#entry2369870
my EVGA Nvidia GT210: works with Inject Nvidia with High Sierra, but no HDMI audio possible
other User with GT710
Zotac GeForce GT 1030 2Gig Web Drivers only  - by @chris1111 Saphire Radeon HD 6670 1 gig - by @chris1111
Tested WLAN:
PCIe: TP-Link TL-WDN4800 N900 WLAN Dual Band up to 450Mbit/s to 2.4GHz or 5GHz, works OOB as Airport
PCI: TP-Link TL-WN851ND 300Mbit/s, works as Airport, thanks Maggo Attached File AtherosWiFiInjector.kext.zip
USB: TP-Link TL-WN725N Nano WLAN USB Adapter 150Mbit/s, not as AirPort
Developer Beta program:
[How-To] macOS High Sierra Developer Beta program without special registration, if anyone is interested!

Copies the com.apple.seeding.plist file to /Library/Preferences/ (not S/L/P)
and open Terminal: sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.13seed-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz see AppStore/Updates  
If the value is in com.apple.seeding.plist = 2, then it is Developers Beta program.
If the value in com.apple.seeding.plist is = 3, then it is Users Beta program.
Users Beta: sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.13beta-10.13-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz see AppStore/Updates 
Credits/Sources - Thanks to all mentioned users:

InsanelyMac.com & root86.com
@Slice and all others for the Clover Bootloader
@vit9696 and @vandroiy2012 for AppleALC and Lilu
@pepto, @polyzargone, @morpheousman, @chris1111, @owbp, @ricoc90 and all others for all Optiplex 780 (760) Guides!
  have fun   Optiplex