It's a good question, actually. Tom's Hardware answered it a few years ago:

Tom's Ultimate RAM Speed Tests
http://www.tomshardware.com/reviews/ram-sp...tests,1807.html

Tom says: "For the vast majority of users, the difference between mainstream and high-end memory turns out to be extremely small."

The tests showed a miniscule boost in performance - 1-3% according to benchmarks. And this was within the margin of error for the benchmark tests, so the boost might have been even less!

So stick with the value/mainstream RAM. It is wonderfully cheap these days. Spend your money on something else that will definitely boost performance, like a better graphics card (If you're a gamer) or a better processor (if you're crunching complex spreadsheets, editing videos, etc).

Also, don't install more than 4GB of RAM if you are using 32-bit Windows (includes XP, Vista, 7). 32-bit Windows AKA Windows X86 can only utilise a maximum of 4GB - this is a software addressing limit; there is no 32-bit way to get past this. The only way you can use more than 4GB of physical RAM is to use 64-bit Windows AKA Windows X64.