Have Spread Spectrum always disabled on P4C800D. Read multiple articles (sorry, can't remember which or where) on BIOS tweaking that mentioned spread spectrum. All recommended disabling the feature as it could be potentially detrimental for overclocking. One article mentioned that the feature had something to do with voltage mods to eliminate interference, and that each board/BIOS handled this differently. I leave mine disabled just to stay on the safe side. Again, sorry I couldn't direct you to the appropriate articles.
There's a short mention of this in a recent MaximumPC. Says the feature varies voltages (CPU and VDIMM, I think) to eliminate electromagnetic interferance. Some BIOS's allow changing the percentages of increase / decrease, but not on the P4C800D.