Don't look at CAS latency as a number that means something by itself. It doesn't. CAS latency in clock cycles
it takes to do the CAS step in RAM. Just because the number is higher, doesn't mean performance suffers. For instance, let's take a look at DDR2-667 @ 4-4-4-12 and DDR2-800 @ 5-5-5-15. And for fun, DDR3-1600 @ 8-8-8-24.
The total latency for these is:
- DDR2-667: 6ns - 6ns - 6ns - 18ns
- DDR2-800: 6.25ns - 6.25ns - 6.25ns - 18.75ns
- DDR3-1600: 5ns - 5ns - 5ns - 15ns
So while yes, DDR2-800 has slightly longer delay than DDR2-667, it's an insignificant performance impact.
Although I'm just going to say to hell with all that. To be honest, I've not really seen a performance impact between DDR2-667 and DDR2-1066. So get whatever you can and what works with your system.