www.w7forums.com has a great article to help you decide which version of Windows 7 to install.
To cut to the point, if you have more than 3gig of ram and don't need to run old software or install old peripherals 64 bit is the way to go.
Take a look here:
I personally have Windows 7 x64 installed on my main development system which has 4 gig of RAM installed. However I end up using my laptop with the 32bit version installed because I have a number tools I need that don't work in the 64 bit version. But I should be honest, I also still use my 5 year old Dell laptop with Windows XP for those really picky applications.