to be honest, I'm not sure weither it'd be worth it or not. I mean, DX10 is nica and all, but does anyone have some(reliable) in game info about how a 8800GTS for example performs in DX10 mode for games? I mean, in 3dmark06 (yea, I know, it doesnt say much) my pc gets a score of almost 8900, wich would be about the same as a 8800GTS on stock speeds(in the same system) if I'm not mistaken. now, My pc cant handle Bioshock fully maxed out, not to mention rainbow six vegas for example. now, how could a 8800GTS run it maxed out in DX10 mode I wonder?
I could be completely wrong ofcourse, but I think its worth looking at it that way. I'm not sure weither getting a 8800GTS (or anything lower) would be worth it when upgrading from a 7800gtx or higher (like your card) simply because you have a card that runs the games pretty good still, and you'd be upgrading to a card that still runs a game pretty good, but then in DX10 mode (probably it'll even run less good than in DX9 mode on your current card.)
but like said, does anyone have some reliable numbers on this matter?