lealla, if your going to go for 2 years, at this point, here is my opinion on a minimum.
4870/gtx 260 level of graphics hardware
4 gb of ram (i'd go for ddr3, however, right now, with a budget in mind, ddr2 is the way to go. that means waiting for amd, and waiting for intel to come out with a mainstream chipset that doesn't have a starting price of 200 bucks)
640 gb hdd
a 500w psu that is 80+ certified from a top tier psu company (this will easily power the system i am suggesting, maybe go up to 650 depending on how much you care about your power bill).
a case with adequate cooling
win7 requires less hardware to run smoothly than vista does (so i've read from beta testing). you can get a 512mb 4870 for 199.99. 1 gb versions run about 250, same as a gtx 260
quad cores start at just over 100 bucks. the Q6600 ($187), which has been out for 2 years, is still future proof imo. its not the most efficient, nor powerful, but amd finally (with its phenom II) produced a quadcore that can outperform it. from a dollar/performance view, it is still a good buy, the phenom 920 for 195 is equally a bargan.
for your motherboard i would hit up either an intel or amd platform. sli/ crossfirex is only worth implementing if you build it from the get go. buying one card, and saying you'll wait 6 months to a year to add annother one, is not worth the dollars/ performance. after a year, the same number of dollars can buy you one graphics card that is equal to 2 of your current one, and uses less power. SLI/crossfire is good, but only with the high end cards, because in 99% of cases, one high power card is better than 2 medium power cards. (the 4850 in crossfire is pretty competitive, cuz you can grab a pair for around 300 bucks, and it kicks the pants off a $250 4870, however, you need to take into account the extra power supply costs)
buy a 2x2gig kit of ddr2 800, or 1066, whatever your pocket prefers (most ddr2 800 kits and 1066 kits are the same chips, just set at factory)
the reason i suggest 640, is cuz its cheap, and fast. the WD 640gig drives are on 2 platters, which make them use less energy, and perform faster than say a 750gig. again, its a small section, 1 TB drives are on par with speed, and in all honesty 99% of people won't notice hdd speed difference.
im stil working the 500 dollar build (i discovered you can get a legitimate dual core processor for as low as 38 bucks)
There is more to life than pew pew you know, like staying out of the fire so you can live to pew pew another day