Hey guys. I'm attempting to install OSX onto my Dell Optiplex 745 as well. The only difference in specs is that it is equipped with a Geforce GT 240 as the video card.
The issue I'm having does not seem to be apparent anywhere else. Basically, when I attempt to boot the installer USB, I can get into the bootloader just fine, but it takes a long time (about a minute or so) just to get into the bootloader.
Once inside the bootloader, I try to load the installer, and the screen goes black. It doesn't start loading the system at all. It's not a graphics issue, because even without graphics support, you can see the verbose boot before the screen goes black. In this instance, I can tell that it can't get past the bootloader because Ctrl+Alt+Delete causes the system to reboot, which does not happen when it is loading OSX.
For reference, I did go into BIOS and set Hard drive to Legacy, turned on usb booting.
To try to get around this problem, I attempted to install OSX to the drive externally using my main system and the installer USB. After doing that, I installed Clover to the hard drive. When I attempted to boot directly off the hard drive, it was the exact same situation. Clover took a really long time to load, and when I attempt to boot into OSX, the screen goes black and does not load the system at all.
I am doing this to install Yosemite, so I'll be cross-posting on the Yosemite forum, and Latitude forums but this is also valid for previous versions as well because I've noted this problem in the past when I tried loading the Mountain Lion installer using Chameleon.