Windows XP Professional, designed from Windows Server 2003 technology, boasts its stability and file access latency reduction as it's big selling points. I run this and its x86 counterpart, and I haven't notice a significant performance increase beyond drive access speed. My low load times seem to be the only impact. I will say that I see far fewer crashes and problems with the x64 platform, and I tend to prefer it for running notoriously buggy games like Battlefield 2/2142. My only challenges were finding appropriate drivers (everything legacy was supported by the manufacturer), and a very small number of slightly older games wouldn't work (received a low graphics memory error). But once I found a solution to any problem, it stayed perfectly operational until I either blew out hardware or made some invasive change that corrupted it. I'd recommend the x64 platform for those with little better to do with their money (or those who have a technet subscription). While the drive access time is significantly reduced, beyond that it's mostly a status symbol, and finding a free antivirus solution can be difficult since only paid versions of AVG support the platform. You wouldn't want to leave this as your only operating system on your machine, per sey, but instead dual boot with both x64 and x86 versions.
I would like to add that I have more success with it than I've had with either version of Vista to this point.