Re: Which one should I get?
Definitely go with the Sempron 3000+. Socket-478 Celeron D's are poor, and are just clock speeds and memory bandwidth (potentially) imo.
The Sempron 3000+ also supports Cool 'n' Quiet, which might be of value to you.
Windows XP Pro 17" LCD Monitor (1280 x 1024)
nForce3 250 Chipset Athlon 64 2800+ w/ C'n'Q
1 x 512MB DDR400 CL3 SDRAM 40GB IDE 7,200rpm HDD (8MB Cache)
nVidia GeForce MX420 64MB PCI On-Board Audio