make sure the motherboard u get is compatible with the processor u get, i just built a computer, it was around 700$ too My Specs- DVD+R/RW NEC Drive, DVD+R Drive Radeon 9600 PRO PC Chips K7 Motherboard 1.8 gigs ram 2.4 ghz processer 320 gig wd 430 w psu and i can run f.e.a.r, battlefield 2, king kong so u can run a pretty good system on a 700$ budget
Go for the one DeF said, just make sure it has the newest technology, so if you have the money later, you can upgrade your videocard, processor, ram etc. (i.e: dont get an AGP mobo, but a PCI-E one.)