CAS latency

im getting two of these modules-
http://www.newegg.com/product/produc...82E16820231048

how far can i lower the CAS latency and what good will it do?
a magazine said it would yield HUGE performance increases.
plus, can i overclock it? cuz my CPU is at 800FSB but my mobo can only support DDR2667

Each drop in CAS latency will score maybe 5% higher in benchmarks.