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You should not spend much on upgrading this system since you will not get more than a marginal improvement out of it. I had (might still have it around somewhere) a P4 660 which I purchased to upgrade an older system much like you have (memory maxed out and new video card too) and it was a noticable difference at first, but I soon realized that it was only a marginal improvement and not even in keeping with the 'improvements' in things like updates and new versions to Windows, Browsers, Antivirus Programs, etc. All of the money spent on improvements, and within 5 or 6 months I was salvaging the new power supply to build a reasonably modern system from scratch that should be good for a number of years still.
A 64bit OS will not run on it. You do not want 64bit XP and Windows 7 would not run on it. It would be slower on anything that is not at least the core 2 architecture (the later Dual Core Pentiums included) with all of it's improvements. Most programs and games do not yet take full advantage of the potential improvements 64bit computing is to offer in the future and those will need a truly modern machine when the time is here (now, for those programs now available).
You have to be a little bit crazy to keep you from going insane.