stay away from that aajs model hard drive preach suggested. that's pure legacy. you want aaks if going wd though i like the 32mb cache 640gb seagate that mav posted. just bought two of them for my media centre.
also, don't even consider the gtx 280 or the x2 4870. as i said above, you need at least a 24" or larger monitor to take advantage of those cards. at the res you'll be gaming at, a 4850 will more than suffice.
for sure go oem on the operating system. there's also really no reason NOT to go 64bit. despite what some say a 32bit system, vista or otherwise, is not capable of using/seeing/allocating/whatever more than 4gb of total memory(hhd and cpu cache, as well as vid card memory). it's a limitation of 32bitness NOT the operating system. yes MS has made vista 32bit "see" that full 4gb but that's mostly to quell the fears of ignorant people who think they got ripped off and only got ~3gb of ram when they paid for 4. yes people were not only rma'ing their memory but their whole computers. that's why ocz (and others i think) came out with 3gb kits. the way vista 32bit interacts with and uses ram is no different than xp 32bit.
preach, ddr2 667?
tek, i've pushed those ram numbers in mercenaries 2. even css pushes my system over 2gb. though sitting here typing this with very little open i'm sitting at 1.63gb used. vista sure is a hog...