Personally, if I had a GTS 640, then I wouldn't bother changing to a GT.
The only reason I say that is that I recently changed my GTS320 for a GT, and although I have gained a couple of thousand 3dmarks in 06, we all know real game framerates is where it's at.
In some games, and on my system, my GT is only on a par with my old BFG GTS OC (eg HL2-Lost Coast/HL2-Ep1), and in others it's about 15% to 20% faster (MOH-Airborne).
It's a very quick card for the money, no doubt, but £180 is still quite a bit of cash, and I personally don't think it would be a very good move, even if you got that amount for your old card.
I don't think you'd be moving very far up the performance tree, to be honest.
Buying from scratch would be a different proposition of course, and the GT would be the best bang per buck.