<a href=”http://blogs.zdnet.com/Ou/?p=440″>Choosing between 32 bit and 64 bit Vista.</a>
If you have a 64-bit compatible processor I recommend you always go with a 64-bit Operating system. 64-bit operating systems are backward compatible with 32-bit applications, so you get the best of both worlds. The only problem is finding compatible drivers for older devices.
Also, 32 bit Operating Systems are not able to map more than 3.2 GB (approximately) of RAM, so if your machine has 4 GB or more, take this into consideration (if you have a processor that supports x64).