I don't own a Sempron, but one website did a review of an overclocked 3100 Sempron, and it preformed better than the Athlon 64 3400. Although hit was overclocked for the test.
http://www.xbitlabs.com/articl...y/sempron-3100-oc.html
(The Sempron 3100 is a socket 754 cpu, unlike the other...