ExtremeTech has tested whether the 64-bit version of Windows Vista offers any performance benefits:
Testing 64-bit performance is still a somewhat dicey proposition. Major benchmarks are either lacking, or don't work properly. For example, SYSmark 2007 simply doesn't run on a 64-bit OS (Vista or XP). And while there's now a 3ds Max 9 SPECapc benchmark, the benchmark crashes consistently with a scripting error before it completes when running on 3ds Max 9 64-bit under Vista 64-bit.
On the other hand, there are more 64-bit applications and benchmarks now. That system-sapping game, Crysis, ships with a 64-bit client. 3ds Max 9, Lightwave 9, POV-Ray, and the Cinebench rendering benchmark all have 64-bit versions. Futuremark's PCMark Vantage offers a 64-bit version of that Vista-centric, synthetic test.
On top of that, anyone using 64-bit Vista will still be running a lot of 32-bit applications. So we benchmarked some of those as well.
The results are mixed, in some applications there was a performance boost but in some applications 64-bit was a lot slower.