In terms of specs I'd buy a GPU with at least a 3TF performance, and even that it's probably going to be "mediocre" for the next 2 years. 2TF should work, too, but might not last you too long, especially for new games. Obviously, the higher the performance the better, so if you can afford to pay thousands of dollars for a gaming rig knock yourself out. It will last longer.
You might also want to keep some accessories in mind, such as the Virtualix Omni or those full body controllers, which could cost you an extra thousand dollars.