32 Bit will only recognize up to 3.5 GB of RAM on your computer.
64 Will go up to 16 GB. However, older CPU's are not compatible with 64 bit.
This information is readily available on the Microsoft Website
Link Removed
Scroll down and you can find out.
Before you download anything, you should know these things.
For your convenience, I will brief it.
If you chose to "Upgrade" you will not lose anything.
If you chose to "Clean Install" you will lose everything unless you have a backup.