The correct memory-speed depends upon the requirements of your motherboard. I’d also guess it’d be smart to make sure it’s all the same. But the bottom line is: if the memory is slower than required, it will (at the very least) introduce unwanted wait-states, effectively slowing the CPU down as much as to half-speed. If the memory is faster than required, there is no advantage and simply slightly-higher cost.
Personally, I don’t “skimp” on RAM. This resource is the single biggest determinant of system speed. I want gobs of it, and I want it to run fast. I’ll “save money” elsewhere.
Provided that the memory is fast-enough for the motherboard, as a rule of thumb you should buy as much of it as you can afford. The reason is, “RAM is the only thing in the system that can keep up with the CPU.” Hard drives are literally millions of times slower. Probably nothing slows down a computer more than paging. The more RAM you have, the less likely it is that the system will slow down for that reason. It’ll obviously have to page material into RAM once, but having done that, it won’t have to page stuff back out to make room for other material during the course of normal, memory- and computer-intensive work (like all 3D-graphics work).