Really depends on what you want to do.
If you're looking for free, then you Linux is your only legal choice. If you want to play many games, the latest and greatest games, etc., you'll want Windows. Out of the windows family you'll want XP. What are your other system specs?
If you're jsut asking for a 64-bit vs non 64-bit comparrison, then go with non-64 bit. There's very little performance gain from using 64-bit, especially if you only have 1GB of RAM or less. As well, there's little softwae out there that takes advantage of 64-bit processors.
Long story short, need more information about you, your computing needs, and the true nature of your question before we can answer.