you cant really build a PC that lasts a long time.
You can get all the latest computer tech out now, and in half a year, one part or more will become "old" tech. At least from what i have seen.
The only site that sells you cheap gaming computers are usually the ones that will rip you off in one way or another.
Just get a Quad, 4GB of RAM, 260GTX (because you dont like ATI), 600W PSU, a mobo with the latest chipsets, and you will have a PC that will last you over a year, and able to max out most games. At least it should.
Far Cry 2 seems to benefit if you have a quad core.