I really want a new GPU now but I hear DX10 is coming out reeeally soon.
January is when DX10 will (hopefully) come with Windows Vista. You'll probably be able to get the first DX10 cards before that, however. Rumours say that the R600 will be a kickass card and the G80 might come out in a few months. The DX10 cards work fine in DX9 mode as well.
As for what you'd use them for, well..they're next gen cards, so they're supposed to be a lot faster than anything you can get now, even in DX9. The first DX10 enabled games should be coming out pretty soon too (Q1 '07, or so), so you'll miss out on a bit/lot if you don't have a DX10 card & Vista. Crysis being (at least one of) the first.
If you decide to buy a DX9 card, it might be a good idea to make sure that you buy a PSU which can handle a DX10 card in the future. Everyone is saying that DX10 cards will require more power.
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