You don't *have* to. Here's the thing. If you buy any of the non-ultimate ones, it only comes with the 32-bit DVD. However, for a "minimal fee" you can order the 64-bit install DVD, and use the same product key (or they give you a new one, not sure which)
However, you can just order the 64-bit OEM version (which is cheaper anyway)
http://www.amazon.com/Microsoft-Windows-Premium-64-bit-Builders/dp/B000MFIPDC - the catch is it must be ordered with hardware, but hell, that can usually be something as basic and cheap as a $5 mouse
EDIT: I just noticed that OEM versions can't be transfered to a new computer (which is a change w/ vista). Not sure what constitutes a new PC, however, so you may get burned by this (eg, replacing the motherboard may be considered a new PC, just like so could upgrading the CPU or any number of other things - I'd watch out for this). Personally, I'd get the UPGRADE edition and just order the 64-bit DVD
As for 64-bit advantages, its still somewhat up in the air. True, it can address more memory (17179869184GB vs. 4GB, theoretical max), but thats not all. 64-bit programs can be faster than 32-bit ones, since they allow for 64-bit division in one instruction. The tradeoff (sortof) is that programs will be marginally bigger, as pointers (which, if you don't know, are used heavily) double in size (they go from 32-bits to 64-bits). But this really isn't a problem, since if you are running 64-bit you probably have 1GB+ of RAM anyway
64-bit will be the future, as consumer PCs are rapidly approaching the 3GB RAM limit of 32-bit CPUs. (whoa, wait, 3gb limit? didn't I say 4gb earlier?
no, thats not a typo, the other ~1gb of RAM space is reserved by the OS, and can't actually be mapped to physical RAM). The speed difference right now usually isn't noticeable, as most programs that are 64-bit currently aren't coded for 64-bit, but rather are just re-compiled 32-bit programs. Meaning they may internally use 64-bit division, but the variables never hold more than 32-bits of information anyway. A true 64-bit program will be noticeable faster. You can see this is the decrease in things like compile times on linux, where 64-bit has been around for a while now