"Beefcake", the Celeron D's are a separate chip. They are based on a 90 nm process; the old one is 130. The Celeron D's generally overclock quite a bit better -- 3.0 should be a minimum, 3.3 to 3.6 with decent cooling, and as high as 4.0. Also, the faster FSB makes a huge difference. And the 256KB cache versus the 128KB cache on your Celeron makes a difference, as well.
Think of the Cel-D's as a faster, cooler-running version of the original 256K cache Pentium 4's.
Both the Sempron and Celeron combo deals are pretty hot, IMO.