It's ok to mix brand types, as long as the specs, such as the speed and timings are identical. I guess it's ok to mix different specs too, but the sticks will run at the speed of the slowest stick. For example:
...you have a DDR 400 and a DDR 500 stick. When you put them together in the same motherboard, the DDR 500 will be forced to downclock to DDR400 speeds, to match it.