I have to agree with Sander,
64-bit is definitely the way forward.
The only software that I have found that does NOT run in Windows Vista-64 was Google Desktop - and that was over a year ago, and I have never been back to see if they have finally rewritten it to work in 64-bit (highly likely now that Win7 is here to stay).
Most '32-bit' software will run in the '32-bit mode' provided by Windows 64-bit OS's, the biggest problem will be applications requiring the use of low-level drivers in order to be able to interface with the outside world. If those drivers were written such that they will ONLY work in a 32-bit OS, then you will need to double-check that they can still work in a 64-bit world, and may need to consider Win7 64-bit as a solution.
Or, as Sander suggests, deem that hardware to be 'beyond its sell-by date', and consider upgrading, or only using it on an 'old' 32-bit platform.
Multiple big images on PI require BIG memory - and that means 64-bit addressing.
Cheers,