My advice is to stick with 32bit for now; it's a lot easier dealing with drivers in 32bit vista right now.
Even in SP1 vista64 requires that all drivers be digitally signed which means that you won't find drivers for older hardware or for hardware from smaller vendors that didn't go through the process of getting it signed which basically boils down to you being SOL.
Vista 64 bit runs Windows XP & 32 bit applications using a function called "Windows on Windows" (WoW) which can cause a performance hit of up to 30%.
The advantage of 64 bit is this:
* Access to more then 3GB of RAM without PAE and in multiple configurations. * Access to multi-teratybe of single point services * Applications that are 64bit of course, benefit.
But yes, there are compatibility issues. Let me give you one that often gets mentioned: Adobe Flash Player (you know, Flash? In websites?) will not install in 64bit Internet Explorer. So, you have to run 32bit Internet explorer to run flash
For right now, unless you have a tangible reason, go with 32bit... it's sad.. MS could have changed this is they would have pushed more.. but they didn't, they bailed.