You have to get Vista for DX10, XP can not do DX10.
Don't matter how close DX10 is, DX10 games that look different to DX9 ones are over a year away yet, all there is out there that has DX10 features at the mo is Flight Sim X, then Crysis when that comes out, but that looks identical to DX9, you do not need to spend £500 on a G80 and £200 on Vista to run those games in DX10, as they look the same on DX9, and will run in DX9 under XP, besides you cant run Flight Sim X in DX10 until DX10 is released (Vista) and you get a DX10 card, but as i said it wont look any different to what everyone is playing it now under DX9 XP.
G80 is only good for DX9, that is all it can be used for until Vista is released, and even then the games in DX10 look the same and work under DX9 XP, so if you have a high end DX9 card now, then there is no need for a G80 as you will only get a few more frames in DX9, and if your high end DX9 card is getting about 100+ already, whats the point, if you can't play games at 100+ you should not be gaming on the PC simple.
Everyone with high end DX9 cards should wait for DX10 to be used more, and worth having, which as i said is about a year away, and a high end DX9 card will easily last, by which time G80/R600 will be well old and have been passed by umpteen faster/better DX10 cards.
If you have a 7900 GTX, and you want a few more frames in DX9, why spend £500+ on the G80, just get yourself a GX2 for a few more frames, its a lot cheaper.