Hi tbl,
Unfortunately, DirectX 10 is only being released for Windows Vista. That means the to get the full functionality of your DX card, you would need to also upgrade to Vista.
Why have they done this? Because game designers have been limited by previous versions of DirectX, and it is the hope the DirectX 10 will break these barriers. In addition, all the processing power should no longer be shared with the
CPU. It's is the hope that all DX processing will take place via the GPU and bypass the
CPU alltogher = better performance.
So I think your question isn't just, do I buy a card that is compatible with DX10 (such as the Nvidia G80), but do you upgrade to Windows Vista?
I remember when Windows XP came out, it was still quite buggy, and lot of of people had to wait for drivers to be released for a lot of the hardware they had. I would personally wait a year or so for Vista's teething problems to be sorted out before upgrading.