Better performance, 1 Xeon 3ghz or 2 P3 1.3ghz?

[Mike]

Right now I run mysql and apache together on a dual p3 1.3ghz box. I
will be moving mysql to a second box, eather an identical dual p3 box
or spending a little more and going for a single Xeon 3Ghz (these are
the only two options I have). I'm thinking of hanging the db box
behind the webserver using a gigabit crossover.

Since I mostly read cached data, I'm leaning towards the dual CPU box
so that when I have a long running query thats hitting the disk, I
most likely will have other queries running on the other CPU reading
from the cache. Is this even what happens? Would the Xeon still be
faster?

Also, I read in the mySQL manual that I'll take a 8-10% hit in
performance when migrating to a second box over using a socket. Having
a dedicated box will allow me to tune mysql to take all the resources
and I assume increase its performance to at least recover the 8-10%,
is that a safe assumption?

I would like some input from wiser people than myself.

Mike