X-bit labs have an extensive 3 part review that focuses first on the best RAM type, in terms of frequency, to use for Intel Core 2 Duo processors, then goes on to look at specific kits available.

Aimed mainly at the enthusiast wanting to get the most out of their Core 2, anyone can see that the gap between memory kits doesn't really translate into anything material. Even in situations where there's 10-15% performance gap between best and worst case scenarios, we're talking about situations where you wouldn't even notice.

It's worth the read just to understand what memory kits are available and how they really perform.

Part 1
Part 2
Part 3