You can do either on the 875p, I lent 1 of my 2 sticks away for a weekend and it was running, but WAY slower. So I assume you could do the same on the 865, but the performance hit is noticable without needing a benchmark or anything. (The slow down might have come when i went from 512 megs to 256megs)
Overclocking doesn't work too well with single channel.
Kevin