what is SDRAM CAS latency? It is under the frequency setting in the BIOS.

will decreasing it overclock my PC?

Yes, it is in your Bios. Basically, lowering your CAS Latency will speed up your RAM a little. If you look on your RAM stick there should be some text stating the type of RAM and the CAS latency. Most commonly it will say 2.5T, but higher performance RAM will have 2T. MIne is cheapy generic, which is rated for 2.5T, not a lot of head room to work with here. If i set it to 2T my system will lock up, so there is a limit to generic, but high quality RAM should be fine.

Say when reading a book you can read a line of 100 letters in one second. You would describe yourself as a 100 letter-per-second reader, but its a bit of an exaggeration because your eyes work at 100 lps only when the letters are contiguous. There is an additional delay when you have to switch lines and refocus on the upcoming line. This delay is what gets reduced as you tweak those memory settings, as memory chips are laid out like the pages of a book except they're made up of bits not letters. These settings can make a 2%-4% difference in memory-intense benchmarks which is not enough to notice as an end-user, especially since memory delays are a small part of overall system delays.

This is a great FAQ page about memory:

http://forums.overclockers.co.uk/showthread.php?s=&threadid=53160