Right now, there's NO support for 8x00 cards. But with the 8600GT drivers from the Santa Rosa MBPs, it'll only take a few weeks till the first 8x00 hackint0sh drivers are fully working!
but right now, at this VERY MOMENT, 8x00 suck for OS X hackint0sh, and so do all cards that have MORE than 256 Mb Vram.
EDIT, just for the record, 512mb Vram cards CAN work if you flash their ram settings, which is not really something for noobs

So if you think you can handle it, try a 512 card ;P