First, you can see if your computer is ready for Windows 7: Download details: Windows 7 Upgrade Advisor
Next, don't activate Windows 7 until you are satisfied with either operating system you choose.
Now... you should consider using the 64-bit OS. You can take advantage of memory above 4GB. 32-bit OS can't utilize the full 4GB.
If you have any older peripherals, you will want to check they will run with the 64-bit OS.
More and more software is being written for 64-bit.
Also, if you install 32-bit, then want to upgrade to 64-bit, you will have to do a clean install.