800mhz & 667mhz RAM not friendly in 2008 Mac Pro?

Hello all,

So I recently acquired an Early 2008 Mac Pro. The system had 6 GB of DDR2 FB 800mhz RAM pre-installed when I purchased it, and I had some 667mhz DDR2 FB RAM lying around that I decided to put in the system. Didn't take my long to figure out that the Mac Pro doesn't like pairing these two. I have tried muitiple different configurations in DIMM slots, but the 800s aren't recognized if even 1 of the 667s is in. All the RAM is good, my question is, is there a workaround to allow the Mac Pro to use both the 800 and 667 DIMMs simultaneously, or should I use the 6 gigs of 800, or should I use the 8 gigs of 667? Any help is appreciated, thanks in advance.

-NM