I thought the limit in the kernel is 64GB, but it's an artificial limit for theorically the limit is far beyond.
By the way yesterday I found this article
comparing Ubuntu running on a x86 and on a x86-64 machine. It's in Spanish, but you may go through Gooogle Translate in order to read it en English. As you can see the tests were basically compilling, encoding and rendering images, this is, heavy CPU work. The conclussion of the article is that, for those tasks, 64bits represents a boost of 20% in performance.
You should have in mind, as well, that if you finally install a 64bit version, you will have to do wether install the 32bits version, or do some hacks, in orther to get Flash and Java running.