the grand parent wasn't promising 2x the computational speed, and neither was I.
As someone who does a fair amount of image processing i'm well aware of single processing/multi processing limits, and how 2x3gh ~= 6ghz, it can be quicker or slower, depending on where the bottleneck is.
eg: the duel processor, quad core mac downstairs is the quickest box i have access to, unless you need lots (>12G) of memory and cpu's (>8), in which case the older altix box (11nodes, 2 cpu's,4G/node)is faster, or if it is a low mem (fits in 2G), sequential job, then my core2 laptop is the quickest box.
Pure Ghz wise, the altix box wins hands down, and my laptop comes in last, but for most serious jobs, the 8cpu's are better, and for minor jobs, 1 cpu wins easly.