The ONLY Sempron 64-bit version in a 2600+ speed rating came in the socket 754 flavor, not socket A and not socket 939.
http://en.wikipedia.org/wiki/Sempron
The 64-bit socket 754 Sempron 2600+ it's a 1600 mhz part with 128k of L1 and 128k of L2 cache, integrated memory controller, SSE/SSE2/SSE3, MMX and 3DNow! instruction set support and full AMD64 support. You're going to see a loss in clockspeed (-400 mhz) and half the cache, but gain performance from a more efficient processor architecture, faster CPU-memory link (800 mhz hypertransport) and a much faster on-die memory controller.
I'm going to say the Sempron should at least equal the Athlon XP 2400 in terms of raw performance with some variances up and down depending on the app. The bigger factor here is the additional RAM - which should have a big impact in a box that is doing web, email, database and DHCP server functions.