Minimal. Top of the line to cheapest value stuff is only a difference of 10% for AMD chips (look, an assumption!). If you've got the cash and everything else in your computer is top of the line then by all means splash out on it, otherwise just get value ram and a faster CPU.
As for matched sets vs. two sticks at random. I don't think stability is compromised or improved for either of them. However two random sticks probalby won't overclock as well or be as reliable when overclocked.