Which operating system is best for my set up?

I just got Parallels 7 so that I can use MS Office on my Macbook (the Mac version of Excel is beyond horrendous for my line of work)

My MacBook Pro is 4 years old. Here are the specs:
Operating System: Mac OSX 10.8.2
Processor: 2.66 GHz Intel Core 2 Duo
Memory: 4 GB RAM (1067 MHz DDR3)
Graphics: NVIDIA GeForce 9400M 256 MB
HD Space: 499.25 GB (115.24 GB free)

My main use for Parallels will be MS Excel 2007/2010 but games would be nice too. In terms of speed, would Windows XP, Vista, 7 or 8 be the best for running Office, specifically Excel? Should I opt for the 32-bit or 64-bit version of the OS?