In regards to this business of mixing ram, I had interesting results mixing Crucial PC3200 with Buffalo PC3200. Both use mT -5B C ICs, but the Crucial could only do 3-3-3-5 >240 and the Buffalo could do 3-2-3-5. I mixed sticks and was surprised to find I could run 245, 1:1, 3-2-3-5 with both sticks! Memtest clean and 3D stable. You would think the ram would only run as fast as the slowest stick.
On a side note, just last night I began experimenting with 4x512 MB ram in my IS7-E, and I swear I can run 5:4 higher up (to the tune of 286, 5:4, 3-2-3-5 at 2.8 VDIMM). You're talking 2 GB ram here. It could only do 280 earlier with two sticks. Than I realized the two sticks I added were actually a better pair (when tested individually) than the original two, so it may have raised the fsb at 5:4!
Moreover, at max fsb where the sticks are borderline, the ram is 3D stable with maybe 2-4 #5 errors showing up. My theory is the errors are buffered by four sticks, and thus are not as fatal to 3D. It seems to me this 865P board prefers to run all slots full. Nothing concrete, just a feeling I have.
At 1:1, not surprisingly I could get 4 mhz higher with 2x512 MB. But my results running 4x512 MB sticks have exceeded expectations. Across the board, 3D performance is significantly smoother with 2 GB (see my other posts, I mainly did this to test the effect on Doom3).