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.
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.
Intel Q9450 @3.2GHz, Gigabyte GA EP45-UD3P, 4 x 2GB G.Skill @4-4-4-12
Antec P160SW case (modded), Xigmatek 750w PSU
3x 500GB (RAID-5), , OptiArc 7200S, ASUS E818A3T
Creative X-Fi XtremeGamer, Hauppauge HVR-1800, Radeon 4890