In spite of what most people here will tell you, for anything but an ultrahigh end setup (like spending $1.000 on just the cpu), or the most ambitious overclockers, money spent on uber ram is just money down the drain you had better spent on a faster videocard/cpu or whatever.
Here is an interesting article that directly relates to your question:
<A HREF="http://edgetechhq.50webs.com/valueram.htm" target="_new"> Value RAM: As Good As The Expensive Stuff? </A>
A small quote from the conclusion:
Investing in expensive RAM probably offers the poorest bang for the buck of any computer part.
= The views stated herein are my personal views, and not necessarily the views of my wife. =