The step up in performance from a 6850 to 7950 is definitely worth it. And if you overclock, the 7950 is definitely worth it's cost, cost/performance-wise. You also get nice games bundled with the Radeon at the moment, so...
Whether it'll hold you over for 3 years is another question altogether. But frankly, to be all set for three years would involve buying a couple of fast cards -- and I don't quite see the point unless you need them for 120Hz or eyefinity right now.
I say, consider yourself good for a couple of years with a 7950, wait for good deal after rebate, overclock the sucker to 1050MHz+, and enjoy.
They're very nice cards, if I do say so myself.