Not much diff for web hosting. Very hard to say how many sites, my experience for such a server is about 2-300 sites. But it all depends on how the sites are running. A busy forum can easily take up the entire server for eg.
Of course there is a difference between the pentium and the celeron. To summarize :
1. Both the Pentium and the Celeron are based on the same core.
2. The Pentium has a lot more cache -- specifically a lot more Level 2 cache than the Celeron (sometimes more than double) which translates into a lot more performance.
3. Celerons are generally slower than Pentiums. What I mean by that is that Intel releases Pentiums at higher clock speeds than Celerons.
4. Pentiums run at higher bus speeds than Celerons, which translates into more snappy performance.
However, with 512 RAM, I would not trust too many sites to either of them. If you have only a webserver running, then it could handle ~500 sites without much grief. However, if you had a webserver, a database server, a mail server all running on the same machine, I would only put around ~300 sites (depending on the kind of sites, of course).
Also, other factors, such has hard disk type/speed come into play.
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