ABIT IB9 and TWIN2X4096-6400C5 - Won't POST

[KeyserSose]

Tried to install additional memory (TWIN2X4096-6400C5) on top of some older Corsair PC5300 memory and the system wouldn't post. After scanning the forum, I get the impression these two sets probably won't mix; however, I couldn't get the system to post with just the TWIN2X4096-6400C5 in place (I did clear the CMOS before trying the 6400s alone.) Any suggestions on what the problem might be? Since the system wouldn't post with just the 6400s by themselves, any chance that these two sets of memory could work together? The only thing I've read that I haven't tried is booting with one stick at a time.