Tough to make a generalized comparison because there are many different models of both P4's and Celerons. Also the comparison will differ based on the type computing workload you are doing. If you have a prescott based Celeron D, then it has 256kb cache - if its a Northwood Celeron it only has 128kb cache.

With all that disclaimer, I would generally say the difference is a few hundred megahertz - meaning I would guess your Celeron 2.6 ghz is roughly equal to a P4 2.0ghz.

I agree with BinaryDemon
I have in my collection 3 s478 computers: Pentium 4 northwood 2.0, Celeron D 310 Prescott 2.13 and a Pentium 4 Northwood 2.26

in all the test's i've made, the celeron d comes out in last place, except when there's a software using SSE3 (which prescott supports, but northwood doesn't), also in gaming there's not so much of a difference, mainly because older games will run just fine in any of these.
later games will show more fps (just a bit more) on northwood due mainly to bigger cache

only far newer games (2007) will show a little improvement on the celeron over the pentium (but that's because of code optimized for SSE3), but still, these games are too new and they expect newer and more efficient processors, or at least older but faster processors (3.0GHz or more, hyperthreading in some cases and big L2 and L3 caches)

i know that these processors aren't the same and the comparison is a little bit wrong or unfair, but i'd say that, for S478 gaming you should go for the fastest p4 you can get, then a middle-tier p4 (2.26-2.😎 and then you should look for celeron (preferably use celeron D)

From my experience:
- Prescott Pentium 4 are not much faster than P4 Northwood with the same clock,
- Prescott Pentium 4 are more power hungry than P4 Northwood - Northwoods are much cooler,
- Celerons D (Prescott) are noticeably faster than Celerons Northwoood with the same clock (more cache, and faster FSB)

So I would say - for socket 478 - go for Pentium 4 Northwood.
Celerons D are not that bad anyway. Old celerons (FSB400/128kB cache) are worst desktop processors of its age, even worse than P4 Willamete.