The sockets could be the same depending on the version of the processor. The 2ghz P4 will definately be socket 478 (or possibly 423 althought that's very rare). The Celeron could be socket 478 or 775 (if its a celeron D). To be honest there is probably little difference performance wise between a 2.6ghz Celeron and a 2.0ghz Pentium 4. The extra 600mhz would probably make up for the fact that clock to clock its gutless compared to a P4 (although I would personally rather have the P4). Thus I don't really think it's much of an upgrade eitherway really. Hope that helps.