There are two ways to approach this.
1. What role is your computer playing - server, scientific workstation, desktop?
2. What type CPU does your computer have - i386, PPC, Sparc?
You can browse distros at http://distrowatch.com/
and then check the requirements for any distro on that distro's homepage.
If you have a laptop or desktop made in the last three years any popular distro can be a good candidate for optimization. Of course, if you don't have special needs you may find optimization isn't needed.