If its mainly for gaming, then yes, i would go with the 1280MB DDR400, since you have more ram, and a lower CL. The DDR533 however would be much better if your using it for anything other than gaming.
Cas 3 is pretty bad for PC3200 400MHz memory, how much are you able to spend here?
If you could get 1280MB 400MHz CAS 2 that would be the best, and you can still get ValueRAM with CAS 2.