The performance gain - even for the applications that can make proper use of 64 bit is not phenomenal it's somewhere in the 10 to 20 percent. Although the subjective increase seems to be enormous to some people
Just leave it at 32 bit.
If you are itching to try it - keep your current install, so you can go back if it doesn't work out. Whenever I try it I find that quite a lot of applications need some sort of special treatment on a 64 bit OS. It's all doable but it's quite time consuming.
