Just to add my 2p to the conversation.
Def go for a 64bit operating system in this day and age. As others have alluded to, 32bit (x86) can only adress 4GB of memory (including the momery on the graphics card). Considering that the majority of pc's now ship with 4gb of system RAM (wil only increase year on year) and 512MB - 1GB graphics card, you need a 64bit (x64) windows to take advantage of that 4GB RAM, since 4096MB (system RAM) + 512MB (GPU) = more than 32bit windows can adress.
64 bit architecture can actually adress up to 16 terrabytes of memory, however Windows 7 has been limited to 8GB-192GB depending on edition.
64bit has come on a long way since I first tried it with windows XP. Back then, very few devices had drivers, which made it impossible to use. Vista improved on this, but some companies are still terrible for their 64bit support (Netgear is one). But Windows 7 has all but solved most issues, as 32bit drivers generally work fine for most things, not come across anything yet that's failed to work.
Same can be said for programs. Basically everything that works on 32bit Windows 7, works on the 64bit version.
There are performance gains when a 64bit program is used alongside the operating system, Photoshop CS4 certainly feels quicker. However not a huge amount of programs have 'propper' versions.
Just realised, you're on a Mac. Oops (hope the rest was usefull for others). Not sure about OSX, but I'm sure Linux has been utilising 64bit for alot longer than Windows, so it's quite possible Apple have had support for a while aleady.