how to compare X Ghz processor to Y Ghz processor?

If I'm comparing dedicated Linux hosting offers, is there a rule of
thumb to tell which of two processors is going to be the faster,
better choice? (Assuming I'm just hosting a busy Web server.)

One hosting company suggested I "upgrade" from a machine with an Intel
Celeron 2.8 Ghz to a machine with an Intel Celeron 430 1.8 Ghz. At
first I wondered why the supposedly better one would have a smaller
Ghz number. But I know that an X Ghz processor is not necessarily
faster than a Y Ghz processor just because X > Y. But in that case,
is there a rule for determining which is better? A published list
that lists all processors in order by their speed (assuming a fixed
task like running a busy Web server)?

Bennett
.

Relevant Pages