Sempron will kick the Celeron's butt. The Celeron is limited to 400MHz bus, but its weakest spot is only having 128k of cache, which effectively cripples the processor. Early P4 Celerons don't have enough cache to feed their long pipelines, resulting in a serious performance hit when cache misses start to occur.
However, the P4 2.8GHz Northwood (533MHz FSB, 512k cache) had a faster frontside bus, and a usable cache, and fits in the same socket as the Celeron. Plus it can be had for a good price used. If you don't already have the CPU's in hand, looking at the P4 2.8 would be worthwhile.
I like AMD a lot...but Socket 754 is even more a dead platform than Socket 478, which you can at least find reasonable upgrades for. Also consider an entry-level Socket 939 Athlon 64 or Sempron, they can be gotten quite cheaply used as well.
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