Another point, keep in mind that you can't upgrade a 7 x86 installation to a 7 x64 one, you have to replace it (well, in theory you could do it but it requires knowledge and, above all, a lot of time to do it properly).
Otherwise it's pretty straightforward, everything 32bit just works, plus everything that has a 64bit version benefits from more RAM and performance.
The only things NOT working with a 64bit version are old hardware with no 64 bit driver (for usb peripherals you can use the Windows XP Mode), 8 (but they never worked on any WinNT) and 16bit apps (but I think they are deprecated in the 32bit version as well, and you can, once more, use the XP mode to use them).
Also keep in mind that despite having more RAM, you won't benefit from it with 32bit apps, as all 32 bit apps won't address more memory and they share the same memory pool (meaning that even if you have 12Gb RAM you wont be able to use more than 2Gb per app or 4Gb among all 32bit apps).
Last point, if you have a fairly recent (and decent) DX10 or 11 graphic card, you really should think to transition to SF2, on Vista/Seven systems it actually performs better than SFP1 with DX9.
Be also sure to have SF automatically deactivating Aero to get better performances.