Pros and Cons:
Windows 32-bit: Con-> can support roughly no more than 3GB RAM(varies by video card used).
Windows 64-bit: Con-> limited support for many older products due to vendors not upgrading 32-bit drivers
Basically if you have any must-have older hardware look into driver support. In all other cases there's no reason not to get 64-bit even if you only use 2GB of RAM.
Windows 8 will be 64-bit only meaning we'll see the phasing out of 32-bit support for drivers.