Not only is there a 64-bit version available, I recommend everyone who has a computer newer than 2005 go with the 64-bit version rather than the 32-bit version.
It's mostly a memory thing. More memory = better performance because your system won't need to access your hard drive as much.
When Windows NT 4.0 came out in 1996, we were all happy with 48MB of RAM. 64MB or 128MB was the complete hookup. 3 and a half years later when Windows 2000 came out, we needed 128MB to run it, but 256MB was highly recommended. Then in 2001 when XP arrived, we found we needed at least 512MB or our system appeared slow. Of course when Vista came out in 2006, 1GB was required, but 2GB was required for the new Aero graphics stuff.
As you can see, memory requirements continue to at least double for each OS. The maximum memory available to a 32-bit OS is 4,294,967,296 (2^32) bytes or 4GB. With that barrier staring everyone in the face, look at what a 64-bit OS can access in terms of memory: 18,446,744,073,709,551,616 (2^64) bytes or 18 Exabytes (18 Billion Gigabytes). A Caveat: Microsoft says 64-bit Windows can only access 128GB, but I have no way to verify that. Regardless, you'll be able to throw as much memory in your box as it can physically hold.
The only major issue with 64-bit OSes are drivers, but over the next year, every major hardware vendor will be creating or testing both 32-bit and 64-bit drivers for all their devices. When the final version of W7 comes out in November or January, pretty much every existing device made after 2005 will have a 64-bit compatible driver available.
On top of that UO runs great on W7-64.
