If you want a problem free system, for gaming, then you're going to be best off with Windows XP Pro for starters. As that's the best supported OS around at the moment.
Vista is still lacking, but moving in the right direction even if it's slow going. The 64 bit OS will be problematic, since the driver support is low at the moment and it's taking a long time for it to catch up, for some reason. Though the biggest problem with 64bit is that about 95% of all software you can use today is written solely for 32bit computing, so you won't get the performance gain you're looking for.
It's a real shame, because 64bit should in theory be much faster and more efficient.
So when all is said and done, you'll be best off with XP Pro 32bit short term, which you might actually have the installation CD for, from your old system (if it came pre-installed then it won't work

).
Vista will be a good option for you in about a year, once all the software companies have patched up their products to run normally in Vista. So far it's not happening too fast, because they're still trying to peddle their latest and "greatest" stuff.
In your shoes, I'd google up whatever you can spot on the internet. All the pros and cons of 32bit, 64bit, XP vs Vista and so on and so forth. Then make up your mind. Don't just jump because somebody here or somewhere else was giving you advice. It's got to be a decision that you're comfortable with in the long run.
Good luck