I gave up on trying to hack this machine into working, in Mountain Lion, three issues are going to be the sound, network and Sata. hacking kext files becomes a pain and just buying these parts is cheaper anyways. the real problem is the 7700 motherboard is 4+ years old and too locked down for non windows users. the kexts that did work in Lion where 32bit and will not work in ML, or at least I was unsuccessful in doing so.
so, here is how I overcame the HP DC7700,7800,7900 short comings. (you must use a supported graphics card)
Sound = http://amzn.com/B0027EMHM6 $3
Sata fix = http://amzn.com/B005B0A6ZS $17 (disable onboard sata, plug this in, works OOB + SATAIII support)
Network = (lots of options here, wireless or wired) http://amzn.com/B0001NYJ58 $22, drivers available for this card from OSX86.net or wireless USB.
NullCPUPowerManagement.kext <-will KP without it
and of course a good DSDT.aml
I have two ML up and running > HP 7700 and Toshiba a205
After tweaking the Bios on the 7700 , every thing went well except sound, tried lots of things. finally purchased usb sound from newegg $10.98 . love it > running Bose 123 > awesome.
Toshiba has been a pain. everything except wireless works. can't tweak the bios. I have to turn off as restart or I have to reset bios to default every time I boot. up.