Guys, I managed to get 9800GX2 work with all resolutions and QE/CI enabled!

To start off, my system specs are as follows:

CPU: Intel Core 2 Extreme QX9650 o/c@3.5GHz - Aircooled
OS: Leopard 10.5.2 (Leo4All)
M/B: XFX Nvidia Nforce 790i SLI Ultra
Mem: Patriot DDR3 PC 10666 @1333MHz
GPU: XFX GeForce 9800GX2 (QE/CI Enabled with All resolutions)
Sound: Creative Soundblaster X-Fi Platinum (Not working with 10.5.2 )
HDD: 2x500GB SATA WD Caviar & 1x750GB SATA WD Caviar

Ok now for the best part of making 9800GX2 work with your Leopard:

1. Uninstall / Delete NVinject.kext
2. Install GeForce.kext, NVDAResman.kext and NVDANV50Hal.kext from here (extracts from latest kexts for 10.5.3).
3. Go to System Profiler and note down the 4-digit Device ID for your 9800GX2 (4 digits after 0x.)
4. Now go to DiaboliK's Helpful Files and install NVKush.

DO NOT RESTART YET

5. Now using some help from DiaboliK's plist generator, we will create Info.plist for the three kexts installed in step 2 using the Device ID for your 9800GX2 noted in step 3.

• On DiaboliK's Plist Generator page, click "NVIDIA". Once you click it, it will expand to show Info.plist generator for GeForce.kext, NVDAResman.kext and NVDANV50Hal.kext.

• Modifying Info.plist for GeForce.kext
  • Let's start with creating the Info.plist for GeForce.kext. enter the 4-digit Device ID noted in Step 3 in the FIRST box saying 10.5.2 GeForce.kext Info.plist. and press Create Info.plist.
  • Now you should see the Info.plist text with an option to copy it to clipboard. Press the button "Copy to Clipboard".
  • Now go to /System/Library/Extensions, select GeForce.kext and go to its contents. Now in the Contents folder of GeForce.kext, move Info.plist (don't use copy command otherwise you will lose the plist text you copied earlier!) to desktop. Now open Info.plist you moved to the Desktop with TextEdit and replace all the text in the file with the plist data you copied to the clipboard from DiaboliK's plist generator website. Save Info.plist and move back to the Contents folder of GeForce.kext.

• Modifying Info.plist for NVDAResman.kext
  • Now we will create the Info.plist for NVDAResman.kext. enter the 4-digit Device ID noted in Step 3 in the SECOND box saying 10.5.2 NVDANV50Hal.kext Info.plist and press Create Info.plist.
  • Now you should see the Info.plist text with an option to copy it to clipboard. Press the button "Copy to Clipboard".
  • Now go to /System/Library/Extensions, select NVDAResman.kext and go to its contents. Now in the Contents folder of NVDAResman.kext, move Info.plist (don't use copy command otherwise you will lose the plist text you copied earlier!) to desktop. Now open Info.plist you moved to the Desktop with TextEdit and replace all the text in the file with the plist data you copied to the clipboard from DiaboliK's plist generator website. Save Info.plist and move back to the Contents folder of NVDAResman.kext.

• Modifying Info.plist for NVDANV50Hal.kext
  • Let's start with creating the Info.plist for NVDANV50Hal.kext. enter the 4-digit Device ID noted in Step 3 in the THIRD box saying 10.5.2 NVDAResman.kext Info.plist and press Create Info.plist.
  • Now you should see the Info.plist text with an option to copy it to clipboard. Press the button "Copy to Clipboard".
  • Now go to /System/Library/Extensions, select NVDANV50Hal.kext and go to its contents. Now in the Contents folder of NVDANV50Hal.kext, move Info.plist (don't use copy command otherwise you will lose the plist text you copied earlier!) to desktop. Now open Info.plist you moved to the Desktop with TextEdit and replace all the text in the file with the plist data you copied to the clipboard from DiaboliK's plist generator website. Save Info.plist and move back to the Contents folder of NVDANV50Hal.kext.

6. Last step, we will change the com.apple.Boot.plist. Go to /Library/Preferences/System Configuration/ and COPY com.apple.Boot.plist to Desktop and open it using TextEdit. Now replace teh contents of the file with the following (change the flags as you desire but the ones I am using were stable for my system):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Kernel</key>
<string>mach_kernel</string>
<key>Kernel Flags</key>
<string>-f cpus=1</string>
<key>Timeout</key>
<string>5</string>
</dict>
</plist>

Save com.apple.Boot.plist and move it back to /Library/Preferences/System Configuration/.

Now go to Disk Utility and select your Leopard volume and repair permissions.

Now reboot and hopefully, like me, your 9800GX2 will be up and running with all the resolutions and QE/CI enabled!!!

Good luck!