if you are looking to upgrade you can get a 512 mb strip for about 60 bucks shipped. You are correct accord man, iff all the memory in the system is a differnt speed, then it will all run as fast as the slowest stick in the motherboard.
*edit*Ill go ahead and shed light on the CL thing
It stands for CAS Latency. The number stands for how many clock cycles goes by before the RAM starts to carry out and read the commands sent to it. So basicly the lower the better.
*edit 2*Make sure you don't go cheap on the memory, for these you get what you pay for....