if there is DDR2 1066 with Cas 5-5-5-15
and then DDR2 800 with Cas 3-4-4-15

Which will be better for gaming?

WinXP Pro 64 Bit
AMD X2 5200 Windsor
Gigabyte M57SLI-S4
Corsair XMS2 2GB DDR 800
EVGA Geforce 7950GT 512

1066 is faster by far.

I read it wrong and answer it wrong too. So get off my case you peanut.

so its RAM speed instead of latency for games eh?

WinXP Pro 64 Bit
AMD X2 5200 Windsor
Gigabyte M57SLI-S4
Corsair XMS2 2GB DDR 800
EVGA Geforce 7950GT 512

The answer isn't cut and dried. If 1066 is faster than required for your system then you should be underclocking it to match the system. When that is done the memory timings can usually be tightened up. Your MBoard calls for DDR2 800. The primary reason to purchase faster RAM is if you are going to overclock your system.

Tight Timings vs High Clock Frequencies

I only know the answer is that the 1066 is better for gaming, but not sure about the details.