MySQL multi-core issue

[Ennio Annio]

Hi all,
I'm experiencing performance issues with a db hosted on a vm: the mysql service does not commit more than 25% of the cpu.
Doing research on the web and in this forum I realized that mysql uses only one of the four available cores because it is not designed to use more (at least my version 5.0.51a-3ubuntu5.4).
I know that at some point in development of mysql multi-core has become supported, in fact, the latest versions have no such problems.
I need to know what is the first version that stably supports multi-core in order to update mysql to that version. And maybe if any of you would give me tips on the correct procedure to follow I would be glad.

Before lifting the usual objections I add this:

I know that the version I'm using is very old and it probably will be the first version that supports multi-core, but I do not want to upgrade to the latest version because I do not need it: mine is a closed environment, the software that interfaces with db is very old, and finally I would not make too much of version jumps: I think I understand that is better to upgrade to a major release version, and then move on to the next until the desired version.

Thanks in advance,
Luke