Ubuntu 64-bit vs 32-bit

You might want to read an article from AMD: Myth and facts about 64-bit Linux

Conclusion from the article:

Use a 64-bit system and stick to the compat layer if you have the need of running certain 32-bit applications.

Tux radar has a short benchmark test that discusses advantages (and at least 1 minor disadvantage) to using 64 bit. Overall, they are for it:

At this point, using a 64-bit distro is rather like enabling hyperthreading on your CPU - you get a free performance boost for your PC, and if that means you can put off upgrading it for another six months then it's an easy win. As we said earlier, it's a nice bonus. Sure, 5-10% isn't a lot, but when it's across your whole desktop and comes at no cost, why not? If you do use an application that isn't in Ubuntu's repositories, check around first to see if other users have any experience of running it in 64-bit.

Unless there is some application you use that is 32-bit only, I really see no reason why not to use the 64-bit version. Even if there are no obvious benefits besides >4 GB memory, I don't think there are any obvious downsides to using the 64-bit version either.