Athlon XP 3200+ only beats A64 3200+ in about one test in a zillion (slight over exageration). Everything bar a couple of tests is much faster with A64 - things like file compressing in WinRAR for example are miles faster. Most encoding tests are a little faster too with A64, but less noticable. The main thing about A64 is gaming performance: the S754 A64 2800+ will beat an XP 3200+ in games, and something like a S939 A64 3200+ will be considerably faster.
That being said though, is it worth paying out for a whole new CPU/Motherboard/Possibly Graphics too, for that gain anyway? If you can squeeze a bit more time out of your XP 3200+, it may be worth waiting for a bigger leap, for example when the 3500+ models become cheaper. If you consider overclocking straightaway, then it would be well worth paying out for. Depends what else you have in your 'rig' really.