From our testing here today, it can be said that Core 2 processors using a default Front Side Bus of 266MHz are fed enough memory bandwidth at 667MHz to keep them running strong in most applications and games. It's not until the bus is overclocked that the need for faster memory is required - this is evident that at 266MHz, DDR2-800 modules don't provide much extra performance, just a few extra frames per second.
Now the question of memory timings, do they play a role? In Synthetic tests the lower timings do help slightly, but in reality, Core 2 is still based on some of the Netburst architecture crossed with the Pentium M architecture. Netburst didn't matter if it was tight timed or loose, bandwidth played the critical role. Core 2 seems to follow slightly in the Pentium 4's footsteps (but not in the processor performance footsteps) - you'll get an ever so slight boost in system performance but it is clear that Conroe likes the higher memory clock speeds.
If you're wondering what to do - tight timings or faster RAM - it's all a matter of preference. Faster clocked memory will allow you to gain some extra bus speed when overclocking (especially if running FSB and memory at 1:1), and that seems to be the big thing, get the most out of your hard earned hardware dollars. Tight timings are something that AMD really has the handle on due to the on die memory controller but it doesn't seem to be too important for Conroe.