Cas is the first number generally 2 2.5 or 3.0
the other numbers are less important but lower is better for all of them except the highest number (i think its TRCD) i've seen multiple tests showing minimal gains if any or sometimes a loss with decreasing that latency. Decreasing it also INcreases the chance of the ram giving errors and causing system stabiliy/operating system corruption.
2-2-2-5 is probably the BEST you could get
i have 2-3-2-6, i don't feel comfortable running 5 TRCD
i say go for some good 2.5 quality name brand ram and u'll be set
but if you're going for best get the XMS line of corsair ram.