Take an inventory of your existing PC... how much can you salvage from it for reuse? Or is it worth trying to sell the PC on Craigslist for some cash?
IF you can reuse your case, dvd drive, HDD, and OS, it can save you some serious cash.
IF you are a student, you can get a free copy of WindowsServer2008r2, then convert it to a DRM free workstation equivalent to Win7Pro x64. read here:
https://www.realitymod.com/forum/f358-s ... -win7.html
otherwise, atm,
i5-2400, i5-2500 or i5-2500k
cheap P67 mATX mobo. make sure it is Rev B3 or Rev B3.1 BIOS
4GB or 8GB of ddr3 RAM 1.5Volt
650W or 700W PSU
and a ATI 6870, 5850 or nvidia 460 1GB overclocked or 480 overclocked (cheap side you can buy a 5770, but I use a 5770 and I'm dieing in ArmA2 for lack of fps)
reuse your case, save $50
reuse your DVD drive, save $20
reuse your OS license, save $35 to 100
reuse your HDD, save $50
Or sell your old PC for a few $hundred on Craigslist.
if you live near a microcenter or fry's, check them out for the best deals.
There are several threads in the forums here in the last month, check them out.
Also, check websites like slickdeals.net and fatwallet.com in their forums and hot deals sections, you can find some vrey good deals on parts if you buy the parts piecemeal.