They have lower latencies.

what he said ^^ and better overclocking potential!

and how do they affect performance?

It doesn't make much, if any difference. Stick to the cheap stuff. PC6400 is plenty. Keep what you have, if it will be compatible with the rig you are building. Although most new computers use DDR3 memory, so you may have to purchase new RAM anyway.

Most so called "high-end" memory modules are designed to be used at higher speeds than your casual SD-DDR2/3 ones. When not used at those frequencies, you might be able to set them to a lower latency setting, which should also give you a little boost in performance for some applications. In general you can use whatever you like however I would stay away from some re-branded memory that you can find in supermarkets, they are sometimes of lesser quality and may fail even without overclocking. The difference in clock speed is not much notable but needed to reach higher CPU speeds without damaging the modules in the process.

Ya anything over ddr3 1600 cas 7 is really to me pointless. You might see 2-7% improved performance but for double the price is that really worth it?

well, 800MHz DDR2 RAM is faster than 667MHz.... the higher the speed, the faster it can access the memory

than i better get a 1333Mhz memory,Higher clocks are not worth the price.
I can get 2 video cards instead

hmm-I recommend getting one good card over 2 cards-Very few cards stack well..

It was just for example to compare memory prices.
I am getting a single HD5850.