Performance wise you may not notice much of a difference with 32 bit apps, but I think there are differences for apps that are optimized for 64 bit.
The performance difference on the 64-bit version is negligible in most games, in orders of 5% or less in most cases. There was a special patch released for Chronicles of Riddick that I think gave it a 3% increase in speed on the 64-bit version of windows.

Vista on the other hand is the pits, I don't have the article handy but there have been benchmarks done post-WHQL on Nvidia cards that are in the orders of 10-40% slower than straight 32bit WinXP. This and all the other negative vibe I'm getting from reading about Vista only cements my position on waiting a good while before getting it myself.

Agree w/you though, it would be very cool to see some x86-64 optimized versions of the TW exe(s) available down the road that offer some real performance increases.