Mixing single and double rank memory? (Not talking channels)

I was flipping through Everest the other day and noticed that the two pairs of memory I have have different rank/bank arrangements. One pair I got about 3 years ago when I first put the system together, with a different board, has 2 rank 4 bank and the second I got middle of last year has 1 rank 8 bank. They were both the same size XMS2 dimms, but the first pair I got had different timings on them. They read 5-5-5-12 on the stick (even though CPU-z and Everest read them as 5-5-5-18) while the newer ones were 5-5-5-18. Both were at 1.8V and I've never had problems running them all at the rated 800mhz at 5-5-5-15 with the P5Q that I have now. I'm guessing it has to do with the way the modules are configured on the sticks, but I havn't seen much more talk about than that. The XMS2 sticks are all 1gb sticks and they all have the same part number.

I was just wondering if running different rank/bank arrangements can cause problems when it comes to overclocking the memory and/or the system?

thanks