Windows x64 is based on a clever gimmick that allows some specially modified 32-bit hardware to run 64-bit software -- with the extra bonus that most 32-bit software can run happily on your 64-bit Windows version
You do need to have a CPU which is x64-capable, but most recent CPUs do have this capability.
The main hurdle would be your hardware, because some of it may not have 64-bit drivers ; particularly if you have any older hardware designed for XP.
The 32-bit is the safe choice ; the 64-bit is the better one, because it's the new standard.
If you don't have any older hardware, and if you don't use any of the old software that may unusually not be Windows x64 compatible, I'd choose the 64-bit ; but if you make regular use of some older XP-era hardware or software, then get the 32-bit instead.
Remember, most old software will work in W7 x64, it's just some unusual applications that won't. Some of these apps are easily replaceable, whilst others would, as you say, cost $$$
Unplugging data drives before installing W7 can be a good idea, simply to avoid systems files being written to the wrong location ; if you're planning some sort of multiboot scenario though, keeping XP as a backup system, then alternative advice would be better.
Check your motherboard and CPU especially ; if they are not 64-bit capable, then you'll only be able to install the 32-bit version anyway.