X-bit labs - Articles - New Budget Processors Comparison: Intel Celeron D vs. AMD Sempron (page 10). Xbit did a nice job of comparing the Northwood and Prescott Pentium 4s with the Northwood Celeron and Prescott Celeron D. They also compared the Athlon 64 with the Semprons (both flavors, the 3100+ is a crippled Athlon 64) and other Semprons (renamed Athlon XPs).
Their net is that the Celeron Ds are much more competitive with the Semprons now, but the Sempron 3100+ is a real bargain. About the same speed as an Athlon 2800+ in real life and about 10% cheaper too.
For me, replacing an old SS51G which has an older socket 478 that can't take the new Prescott Pentium or Celeron D, it says that the older Celerons are about 1/3 lower with the same clock. For instance a Celeron at "Newegg":http://www.newegg.com/app/ViewProduct.asp?submit=property&DEPA=0 had a pricing like this:
* "Celeron 2.6GHz":http://www.newegg.com/app/ViewProductDesc.asp?description=19-112-178&depa=0 for $88, so it is equivalent to a Pentium that is about 25% slower (2GHz Pentium)
* "Pentium 2.26":http://www.newegg.com/app/ViewProductDesc.asp?description=19-116-142&depa=0 at $118. So for 34% more, you get about 10% faster.
* "Pentium 2.4B":http://www.newegg.com/app/ViewProductDesc.asp?description=19-116-140&depa=0. for $127. This is a NOrthwood running a touch faster (about what my old one was). That's 6% faster for 8% more money.
* "Pentium 2.4C":http://www.newegg.com/app/ViewProductDesc.asp?description=19-116-158&depa=0. for $137. This is a slightly update Northwood with 800MHz FSB vs. 533MHz and aHyperthreading. Unfortunately my Shuttle SS51G can hyperthread but has a slower bus. Hyperthreading by the way doesn't clearly help performance (but it is kind of cool!).
Net, net right now at the ultra low end, the best price performance is the Celeron 2.6 which is slower than the Pentium 2.4GHz that was originally in the machine (by about 1/3). If want to get the same performance, then the 2.4B is the ticket at $127. I think I spent about $200 for the thing when it was new, so prices really haven't gone down all that much in the Intel world really in the last two years.