I am using WINXP64 and it does improve performance but not much can be seen in case of 32-bit applications. I work in MAYA and wanted to use 4GB memory, so switched to 64-bit OS, as more memory does help me in working with maya. BTW, I am using 64-bit version of MAYA.
Till now haven't faced any problem with driver compatiblity, games not running or stuffs like that. In a nutshell, it does improve performance but if you are not going to use more than 2GB RAM then stick with 32-bit OS.