When it says it works with DDR2 1066 it means that it will be able to take full advantage of memory of that speed. 800MHz memory should still work, but the motherboard will have to slow down the memory bus for it and you won't get the same memory performance as if you used the faster memory.
When it says it works with DDR2 1066 it means that it will be able to take full advantage of memory of that speed. 800MHz memory should still work, but the motherboard will have to slow down the memory bus for it and you won't get the same memory performance as if you used the faster memory.