Well the main thing is what games.
Some, like Oblivion, will see dramatic improvement, others will only see slight improvements.
As for the DX10 component, there's nothing really compelling yet to say how it will handle DX10, and whether it would be worth the upgrade. IMO in games that have a DX10 option, the DX9 path will be very shader intensive like Oblivion (as we saw in COH) and that type of game will play well to an 8800 even in DX9 mode.
If you don't NEED to buy now, then I'd say wait, but if you do play games like Oblivion, then check their benchies and see if the performance or settings difference (Oblivion noticeably adds HDR+AA and better AF filtering) .
There's places it will shine and others it will be only mildly different. I'd say if you don't get a compelling reason for yourself, then wait for that compelling reason.
As always, just my view from the cheap seats and my two frames' worth.