64-bit is mainly good for two things.
One, your memory limit effectively goes away. With a decent graphics card, 32-bit Windows clients in reality only give you a maximum of about 3-3.5 GB usable RAM. Plus individual 64-bit applications aren’t limited to being able to use 2 GB RAM as most 32-bit apps are under Windows.
Two, some programs written to take advantage of 64-bit will run faster. The 64-bit chip has 64-bit registers (ultra-fast local memory) instead of 32-bit, and it has double the number of registers in the core and in the SSE unit. These in themselves have shown IIRC a 20% speed bost in some games.
I haven’t verified this, but it is possible Windows itself runs faster as Microsoft probably recompiled it to take advantage of 64-bit processors.
The downsides are 64-bit programs take more memory and disk space and you may have compatibility problems with some older programs.
It also appears none of these improvements will benefit you. But then if you eventually decide to do heavy Photoshop or gaming you’re going to wish you had bought the 64-bit OS.
Not to mention, there is NO FLASH for 64 bit yet! (OK, there is Gnash for Linux, but I haven’t been able to make the Windows 64 bit build work) Therefore, you’ll be reduced to using a 32 bit browser if you want to see the content on 95% of the web.
Cool. Luckily, I think the upgrade has both 32-bit and 64-bit versions together, so if I ever do need to switch I can do so for free.