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.
Q6600 G0 @ 3.80ghz
MCR-320-->Dtek Fuzion V2-->Dtek Fuzion GFX2-->Swiftech MCP-655-->
Asus P5K deluxe
4x1gb Crucial Ballistix @ 846mhz 5-5-5-15
3x 250gb HDDs
Coolermaster 690 w/ water cooling mods
3dmark06 - 15,602
SuperPi 1M - 12.421s