As I've just spent the better part of 2 weeks working on this, I figured I'd chime in and help anyone out that I can.
I now have Mountain Lion 10.8.5 working FULLY on a Dell Optiplex 745 ...iMessage, QE/CI, Audio, Graphics, the works.
This was my boxes config:
Intel® Q965 (ICH8) Express Chipset
Intel® Core™ 2 Duo 1066MHz FSB Socket T with Dual Core technology XD, EM64T, 2MB and up to 4MB L2 cache, EIST and VT (E6000 series)
Broadcom® 5754 Gigabit Ethernet LAN solution 10/100/1000 Ethernet with Remote Wake Up and PXE support
128MB ATI Radeon® X1300
ADI 1983 High Definition Audio
Video Card Notes: The onboard video card's VGA works for all installation needs natively but sucks and doesn't support QE/CI..I saw some places that you could but it involved a lot more than I wanted to do.
So, I ordered this ATI Radeon HD 5450 1GB for $30 - $20 rebate = $10...SOLD!
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131338
My original install was all done through creating a MyHack USB with IOATAFamily.kext in the Extras folder on the USB Drive (Otherwise, Disk Utility won't show your SATA drive to install on)
iMessage was by far the biggest PITA to get working...so I had to rely on a relatively new bootloader called Clover to get the job done.
I'll upload my kexts/extras folder stuff if anyone still needs this, just reply to the post. I love this forum and this was a fun little project for myself so I'd be glad to help if I can but the main takeaway is...get a different graphics card.