Athlon XP3000+/333fsb version or Sempron 3000 which isn't quite as fast but runs slightly cooler.Both have Barton cores so will be better performers than your 2400 Sempy.
You'll certainly notice the difference in speed but it may well be more economical to just get more ram if you have less than 512mb's at present.
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1100t on M5A99x-EVO with 2x4gb and 6870 running Win7 on Force120 ssd in Corsair 600t cooled by H100. Folding for team 34106