I have been very disappointed with Crucial products lately. Of the last 3 builds I have had one stick out of 3 different kits go bad. 2, 2gb kits of the PC6400 from the Ballistix line, and 1, 2gb kit 0f PC8500 also from the Ballistix line.
The PC8500 was returned to Crucial for replacement and the new kit has been running fine for about 3 weeks. 1 of the PC6400 kits was returned to Newegg and the replacement kit has been running fine for about 2 weeks. The other PC6400 kit will be returned shortly. No hurry on that one because I had an extra kit of their Tracer line at the same specs that I had lying around.
I have used Crucial almost exclusively in all my builds and never had a problem. Maybe my luck has run out? Or, maybe Crucial had a bad run? I will give them one more chance before switching.
By the way, all of this memory was running at specification. No overclocking.