Is higher or lower CAS/latency better?

Lower is generally better (4-4-4-12 vs 5-5-5-18)
Tighter latency means that it takes a lower amount of ns (nanoseconds, the measure of speed for RAM) to transfer data. However the frequency/transfer rate (DDR2-1066 v DDR2-800) can make up for a higher latency, which is what you tend to get as RAM gets faster and faster (DDR-400 was 3-3-3-8 standard, DDR-533 was CAS 4-4-4-12 standard, DDR2-800 is 5-5-5-18 standard)