The Celeron D (Prescott)is a good value low end processor, and its performance is 15-40% better than an equivalently clock Celeron (Northwood).
In real life benchmarks, it outperforms the AMD Sempron, except for the 3100+(which has a much superior core to all of the other Semprons)
http://www.xbitlabs.com/articles/cpu/display/sempron.html
I've built a few Celeron D's for people lately, and they are very good overclockers.
e.g.
Celeron D 320 (2.4G, C0 stepping) o/c to 3.1G, stock fan & voltage. Underload temperature about 51C, in low ambient room.
Celeron D 320 (2.4G, D0 stepping) o/c to 3.5G, stock fan & voltage. Underload temperature about 50C, in low ambient room.
Celeron D 330 (2.66G, C0 stepping) o/c to 3.4G, stock fan & voltage. Underload temperature about 51C, in low ambient room.
They are also very, very good for video encoding MPEG2s.(for DVD)
See:
http://www.xbitlabs.com/articles/cpu/display/celeron-d.html
Page 13 gives you the run down on video encoding.
Cooling is not a big issue with Celeron Ds even though they're Prescott based. It is probably due to the fact that the cache is only 25% the size of a P4 Prescott, at 256K vs 1M. The stock heatsink is also a good quality unit with a copper core.
D0 stepping Celeron D appears to be have slightly better thermal and moderately better O/Cing ability. I had the Celeron D 320 2.4G, D0 stepping running at 3.6G (800MHz fsb) but it was giving errors in Prime95 about every 15-40 iterations. (which was not bad, but not fault free) A voltage tweak or more extreme cooling may fix this.