Core 2 Duo is designed to work with DDR2 533 RAM. It will not slow down your system. On the other hand if you want to overclock, then you may run into issues where you cannot increase the frequecy past a certain point.
I have a E6600, ASUS P5B Deluxe and Kingston Valueram. The meory is DDR2 533. My question is, will this memory cause the other components to not run at their optimum speed?
Having faster RAM can still yield higher speeds, but with a Conroe system the benefits may not be worth the cost unless you plan to overclock. I know for sure that RAM with lower latency offers little gains with Conroe. I believe there are larger gains with RAM that runs at higher clock speeds, so DDR2 667 or 800 with Cas latency 4 or 5 may be a better choice if your looking to upgrade.