you used to be able to exchange your license for x64 due to the fact that microsoft kept delaying it and not complying with their agreement with amd, which stated that if you make the processors, we will make a version of windows to support it just like we did with itanium.
that exchange policy has since ended, so no you can not do it any more. However, you can go to pretty much any computer builder store and buy a copy for the same price as professional full.
Although, under x64, you will not see any performance increase unless you are doing database work or playing one of a couple games that actually support native 64-bit code execution (UT2004 and Farcry). In those games, your performance increase will be about 15-20%, however all of your normal 32-bit games will be running in 32-bit to 64-bit conversion emulation, making them usually perform about 20-30% slower than they would under the native 32-bit OS (this is something that microsoft has resolved in the most recent build of vista). In the latest case, 32-bit applications can run up to twice the speed in vista x64 due to the fact that they have a conversion layer that instead of writing zeros to fill in a 64-bit calculation, they take two 32-bit calculations and stack them in the same cycle.
If you want x64 in Windows, you should really look to Vista because it's implementation is really much better than XP x64.