The Celeron D 340 In my computer has a multiplier of 22!! That would be like 4ghz if you pushed it. Im gonna look for some benchmarks for the D's overclocked.
I was able to overclock my celeron d from cpu freq 133 - 155. It runs very smooth and cool. That goes from 2.66ghz to 3.10ghz, which is about a 17% overclock, which is a big difference.