Intel hasn't released Celeron versions of Core Architecture Conroe processors while Conroe's are going to have 1333MHz and higher FSB's soon. That means all current Celeron processors are seriously outdated. They are all single-core. Of them, Celeron D processors based on 65nm manufacturing processes are a little better than the "original" Celeron processors based on Northwood and Prescott processors. For some things, Celeron 2.8GHz with 400MHz FSB and 128KB L2 should do, but there are just too many processors that cost under US$50 and still do much better than them.
I don't think it's important to tell whether Sempron 2800+ is better than Celeron 2.8GHz.