You have to have a 64-bit processor to run a 64-bit Operating System. Look up your processor on wikipedia or something and you can tell. A lot of processors these days are leaning 64-bit, but are being shipped with 32-bit systems. This isn't always the case though, so be sure to check up on it.
Only 64-bit Vista is shipped with 64-bit processors. I've kept a pretty close eye on the mass produced PC's out there and they're all mostly 32, except for a couple systems with 4 or more gigs of RAM. Those all have 64 bit processors.
So anyway, Naz, go with Windows 7 64 if you can, or Windows 7 32 if you have to. If you can't do either, do Linux before you do Vista.