Knowing this and given:
A) You want to fold
B) CUDA Support
C) Only Upgrade for a bit
I would go with the video card. Reasoning is:
If you go with a MB only yes you can still fold but you can't use CUDA with the 7800 series. Also and this is small since your getting vista and have an Interest in DX10 you also lose that. So this is why I say it just based on the points that you brought up.
If you were going to upgrade later I would say MB, then video card, then CPU. This is not the case however and you have a few specific things that you want and the MB only upgrade would not give you. The Video Card will however provide those.