Well, DX10 is a nice upgrade over DX9. Beyond that, I'm a bit fuzzy on the details. I *doubt* it's going to be required for awhile. While we can hope that all games would be as flexible as HL2, I wouldn't bank on it. There's a lot of speculation along with the facts out there, and I simply haven't been able to sort through it all. I can't see the game industry automatically flipping over to Vista/DX10 only support just yet. Some of this stuff, I'm just as confused as the rest of you.
What I do know is that DX10 is going to offer some major benefits. Now, stuff like Alan Wake, I'm not sure off-hand if they're really DX10 shots or not.
DX10 steps up from what I've been reading, and I would hope so, given that each subsequent release of DirectX adds extra features. The real-time day/night and weather conditions are supposed to benefit from DX10, so Crysis and Alan Wake will look better. I suppose there could be a nice little addtion in the control panel if you have DX10, but I'm only speculating.
The 7900GS is a nice card, which I'll hopefully have a review up soon (as in Christmas-shopping time).