I'm overclocking a Linux box (SuSE 10.1) from BIOS only.

What's a good Linux app for measuring my gains from the OS after rebooting? (I'm not interested in any tools for overclocking from the OS). Just need a tool that I can use from my OS to check my BIOS results.

You might look into 'nBench', 'GeekBench' or 'interbench'.
I use nBench myself, so I cannot speak for the other two, except to say that GeekBench doesn't come in a version for Slackware so I couldn't get it to work for me.
nBench does 10 different CPU/Memory speed tests, and while it is NOT a super-high-tech application-extraordinaire, it is useful for making comparisons of your system after making changes to FSB, memory timings, etc. I use it regularly after kernel compiles and memory speed adjustments.
You can likely find each of them and more, using Google or SOurceforge.