Ah ASUS P5B and Corsair memory
What have you got the memory set at in the BIOS...DDR2-800? Take the RAM off the auto setting if you have it set at that as the P5B reads the SPD info off the RAM incorrectly. Try a lower divider like DDR2-667 or even DDR-533. What Cas rating is it, Cl4 or Cl5? I have the same speed RAM in one of my other rigs and it was a right royal pita to get running properly. Look at your timings which should be 5-5-5-12 according to Corsair, but they prefer 5-5-5-15@ 1.9v. Failing that, head over to Corsair's forum and check out the compatability questions for ASUS P5B and XMS2 PC2-6400 here
I'm betting that the RAM is flakey and stuffed or running the RAM on a lower divider will fix the issue. If you are required to run it at the lower divider then RMA it, as it should run at DDR2-800 at those timings.