Since you have 4GB of RAM I see no reason to install the 64-bit version.
As far as can you ? I can't be sure but I'm gonna guess not on the early 2009 Macbook.
If you do you will need to find the correct drivers and install them manually since bootcamp won't support the 64-bit version.
It's possible, and I did it myself. You just have to go into the command line and type in a couple things. There are a few threads on here about it, just search it