well, by statistics, $900 is considered a budget or a midrange computer. you cant expect to build a pc that will play the highest end games smoothly wtih a budget like that, especially without overclocking. the radeon 9800 pro shows lag in farcry at full settings, and just wait until half-life 2 and doom 3 come out. in a month or 2, the radeon 9800 pro will technically be downgraded to just mid-range performance. also, i understand about your budget, so 1 gb of ram is not realistic right now (who says you cant add more later?!

). also, if you are willing to downgrade from 64 bit to 32 bit, the strongest performing athlon xp is hte 3200+. its costs $187. the athlon 64 2800+ also costs exactly the same for a bit increased performance, so stick to 64 bit anyways. not to be pessimistic, but even if you can get the 3000+ and the radeon 9800 pro now, it will be outdated by next year. expect athlon 64 4800+ and a radeon x--- xt to be out by then, and games will only become superior to doom 3 and half-life 2 then also. in my opinion, you should wait til mid-late summer to build the comp. if you desparetly want it now, then i totally recommend you overclock. as for putting in a mobile cpu into a desktop, its perfectly fine, especially with overclocking. low voltage, low temps, and low fsb benefit overclocking, so wtih a good psu and motherboard, you can expect to catch around 2.5ghz on air. in benchmarks, 2.5ghz is near athlon fx-53 speeds, expecally in games. i'm not saying you still cant go for an athlon 64. you can still stick with:
amd 64 3000+ $223
mobo $99
mushin 512 pc-3200 $90
antec sonata case w/ 380W psu $99
radeon 9800 pro $210
seagate 120gb sata hard drive $97
total: $818 w/o tax @ newegg.com
total should end up being $900 or so, but until then, this is the best you can get for your money retail wise.