well i run the 4870x2 in EQ 12gigs of DDR3 Ram and I have everysingle setting cranked to the max on 2 22" monitors with 2 accounts with no problems what so ever.
My Opinion if your gonna spend some money to build yourself a new PC, get the best you can afford. The technology out nowadays is so blisteringly fast that it will not be outdated for a long long time, Software is not even close to needing the power of what is currently offered in hardware for a very reasonable price.
ALso,
I would recommend building it yourself it is the easiest thing to do and can save you alot of money, esp when you are building a quality gaming system with some higher end components. 1500-2000 will get you a 3-4k PC/.
just my opinion, im no genius.
Look into Newegg, they offer 12 months no interest and have some of the best prices on the web.