As everybody says the lower the better but here is a layman's explanation about those numbers.
Imagine your memory bank in a grid with rows and columns. Each intersection point representing a location or address in your memory chip.
The computer will find those address by enabling a row and a column and where they meet is the addressed seeked by the cpu.
Depending on the quality of the RAM, it will take a certain number of clock pulses to address the proper row or column. CAS (column address strobe) is a signal expressed in number of clock pulses to find the proper column. RAS (row address strobe) is the same thing but for row.
CAS to RAS or latency time is the number of pulses required by your RAM to pass from a column to a row signal.
It is easy to see that the lower number of pulses required means faster RAM access time therefore better performance.
My gaming machine:
P4 2.8 GHz
2 X 512 MHz PC3200 in dual DDR mode
Intel D865PERL MOBO
integrated 6.1 MAX audio
Logitech duo MX mouse and KB
ATI RADEON 9800 PRO