Well, I think, like everyone else here, that your best course of action will be one that, in the end, spends the bulk of your money on a nice, shiny set of DirectX 10 cards. I'm not quite sure when they'll be hitting shelves, but likely well before Vista will actually hit the scene; hence, their DX 10 features will be dormant until you upgrade Windows.
As for right now, I see two good options: the first, suggested by prozac26 and GGA, would be to go for a good, fairly high-end card; the Radeon X1800XT and X1900XT seem to cover this spot the best.
Another, possibly better, would be necroshine67's suggestion. eVGA certainly DOES offer a "Step-up" program; if you provide them with reciept of purchase of one of their video cards, within 6 months of purchase, (it might be 3, as I don't remember the exact amount) they'll give you full credit for the price you paid, which can then be applied to any new eVGA card.
Hence, you could go for a 7900GT (or even a 7950GX2, if you feel compelled) and then later, once the new cards launch, trade up your card for a new one. (or two)
The disadvantage of this is if the cards don't launch before the "trade-up" duration expires, you're left to sell off your old card the old-fassioned way, over eBay or what have you.