Just curious as to why one would use a Sempron when single-core Athlon 64's can be had for dirt cheap prices these days? At current prices, a new or used single-core Athlon 64 probably wins the bang-for-the-buck race. Unless of course you already have both of these CPUs.
EDIT: Just saw a posted deal on another forum --Newegg has the Pentium D 805 (dual core, 2.66GHz per core) for $65 with free 3-day shipping. If this is something you're contemplating as a system build, this option would let you go with a Socket 775 mainboard so that you could later upgrade the system to Core2 if you wanted. Just figured I'd post it.
Link here
In the same vein (perhaps a goodwill-towards-all gesture?) they also have the Socket AM2 AMD Athlon 64 X2 3600+ for $65, free shipping, and an offer that gets you an OEM copy of Rainbow Six: Vegas free with purchase. The AMD chip runs cooler, and is probably faster than the Pentium D.
Link here