I think it's ~25% better on average. IMO, not worth the upgrade, since it doesn't cost below $100 and you will need a new mobo (and RAM, if you don't have DDR400).
However, getting A64 3200+ (2.2 GHz, 512k L2 cache) should give you ~40% performance increase (on average) and I think it's good upgrade, though the CPU costs more.
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