Windows 7 Forums have a new article which helps explain the pros and cons between 64 bit and 32 bit versions of Windows 7. You can check it out over here.
"Not only are there going to be several Windows 7 editions, but you will also need to decide if you are going to run a 32 bit or 64 bit version of the operating system. If your CPU is more than a few years old there is a good chance that you can only run the 32 bit version of Windows 7, however if you have a modern processor you may have to decide between each version.
The 32 or 64 bit architecture refers to the memory address length that can be referenced by the processor. This also has an impact on the maximum amount of memory that can be utilised, which is 4GB for a 32 bit CPU (but in reality the maximum accessible ram is closer to 3.4 GB due to other hardware allocations)."