I had windows Vista and put windows 7 on it. mine is 32bit and I've had absolutely no problem with the games other than Millenium 2 and my characters locked going in one position 3 times in one night. Each time I had to reboot the computer.
Depending on your machine and what you had on it is what decides wether or not you can put 32 or 64 bit. From what I can tell, 64 is supposedly better for gaming, but there are less problems with 32 bit.