I've used Corsair XMS of late and haven't had any problems with it.
To expand on what Sazar mentioned...the CPU's clock is determined by both the front side bus (which is the clock the RAM runs at), multiplied by the multiplier. So in this respect memory clock can effect CPU clock... If the fsb is a lower clock then the CPU was meant to run on, then the CPU will clock lower unless the multiplier is increased.
I had this happen with my current system when I first got the mobo. For whatever reason, with an AXP 1900+, and DDR 300 MHz RAM (Corsair XMS 2400 CAS 2.0), the board (Asus A7N266-E) would auto-detect it as DDR 200 RAM (100 MHz x2 for DDR) and ran the CPU at 1.2 GHz instead of 1.6... For me, there was an easy fix...set it to manual, and bump up the clock to where it should have been. Latter BIOS revs on my board didn't have that prob though.