I had a XP2500+ also with FX-5900XT, 1 GB RAM, etc. The upgrade to the AMD64 3200+ has been better than I tought, not only because the speed improvement (which is also greater than I thought), but also because it runs cooler than my Barton so it's also quieter
. I can only recommend the upgrade; it will also guarantee a much longer life-span between motherboard upgrades, which is the most painful one (if only because you must reinstall the OS and everything from scratch)
.