Forseeable future?? IM hoping atleast 1.5-2 years from now...will tat be possible?
But seriously... no way. There has never been a VID card to last this long. You'll be able to play, but not on HQ AA/AF. Before you start dreaming think of past high-ends: 9800pro, 6800ultra, 7950GX2, x1950XTX... 2 years ago you could have chosen between 7950GX2 and x1950XTX, both highly able to play any game at high settings at that time, what of it now?
The differences are made between architectures, not between the midrange and the high end of the same architecture. From the pow of next 2 year's games it will be quite the same thing if you have a 3870X2 or a 3870, these games will be playable at low settings and/or low resolution.
Sorry to spoil for you mate, but it wouldn't hurt to research a bit before saying "I'll spend 500$, but at least I'm covered for the next 2 years". Sooner or later, usually after a year, you'll start moving the sliders towards the left par of the screen to keep those FPS.
My 2 cents.