I have a HP DC7900 with an ATI HD5670 GPU successfully running retail Lion the only extra kexts being fakeSMC, VoodooHDA and AppleIntelE1000e. I have made several modifications to the DSDT most important being HPET which stopped the USB ports working. The only remaining item being sleep not working, it goes into half sleep, though fans still running then it will not wake.
I did previously have it working, sort of under Snow leopard 10.6.8 using sleepenabler, although the first time it went to sleep it woke immediately, after that it worked perfectly. With Lion I get kernel panics when I use sleepenabler regardless of pmVersion settings.
If nothing else hope my DSDT helps someone else getting a HP DC7900 running, you do need to add a supported graphics card though. I have also included the original DSDT extracted from Win7.