CL = cas latency. I think that is the most important of the latencies. If u want to know more use wikipedia.
mhz = mega hertz. It's like with processors, the faster (more hz) the thingy has the better it performs.
U already have 512 so if u add another one it will be fine. But I'd still get that 1GB because the more the better. And 1GB ain't much anymore. That's why I'm getting 2GB
oh, and use the edit button instead of double posting.