Upgrade to dual-core (Pentium D) processor - Does it worth it?

[Tobias P. Santos]

Hello!

We have a server running FreeBSD 5.4 i386 (which we plan to upgrade to 
6.1) with a single Pentium 4 2.8GHz processor without SMP kernel.
This machine basically serves web pages that may or may not use MySQL 
server (also run in this server).
Usually, we have low usage and load average is ~ 0.2 which is ok, but 
sometimes, one single process (e.g., mysqld) begins to use all CPU 
cycles, loading up the system and leading the other smaller processes 
(in terms of CPU usage) to have a high response time.
As this server's motherboard doesn't support an additional processor, we 
are wondering whether a Pentium D processor (830) might help to minimize 
this problem.
In our understanding, one core would be busy with mysqld for example, 
while the other one would be free to handle other processes like httpd 
and serve pages that don't even need mysql resources.
Is this correct? Did someone have similiar (successful or unsuccessful) 
experiences?

Thank you in advance,
Tobias.