BTW, just to test my theory I tried something with my gaming rig:
I've got a heavily-OC'd and liquid-cooled AMD/ATI setup (Asus Crosshair V, Phenom II X6 1100T @ 4.7ghz, 32GB DDR3-1600 RAM, 2x Radeon HD6970's OC'd, 2x240gb SSD's, 2x120GB SSD's, 1x240GB PCIe x4 SSD, RAID10 2TB 7200rpm HDD's, external-enclosure water cooling w 3 main 360mm radiators with 3push/3pull fans each, 2L refrigerated reservoir, 3 main pumps, 3 inline pumps, 3 inline 120mm radiators, 1/2" tubing, pure-copper heatsinks, etc - resulting in 3 separate H2O lines and temps that STAY at ambient unless pushed hard).
Just to see what difference the RAM makes on its own, I took out 3 of the 4 8GB sticks and ran PCMark. Then, I took out the last 8GB stick and tossed in 2x4GB DDR3-1600 RAM sticks (both are 4/8GB sticks are G.Skill Ripjaws X series, timings were set identically) and ran the same exact PCMark test with the ONLY difference being dual-channel versus single-channel.
423point difference overall, in favor of the dual-channel. There are likely many more variables at play, seeing as how I have a processor running 1.5ghz higher than normal, both GPU's running almost 200mhz faster than normal (with shader and memory speeds increased by ~400mhz and ~1ghz respectively), while NB/SB on the MoBo are liquid-cooled and overclocked, as is the HyperTransport-Link.
Clearly, dual-channel versus single-channel will have noticeable performance effects, particularly combined with a dual-core processor, assuming that the memory controller is optimized to take advantage of dual-channel DDR2, which is 4x effective memory bandwidth.