Do not switch your OS unless you intend to run everything as 64bit.

I am not totaly sold on the idea of going to 64 and would stick it out to the end tbh. I agree with Matt aswell, i have used Vista in the past and will not change anything over until Vista SP1 is released (as all the major bugs will have been dealt with - yes SP1 will bring it own, but hey the joys of an MS OS).

If you really feel the need to go 64 and up to Vista x64 then do it, but be aware of any additional cost you may end up with.