Benchmarks: PiFast, Cinebench and PCMark
An Intel Core i5 2500K processor can calculate Pi to 10m places in next to no time, and contrary to what may be popular belief; more RAM doesn't mean more speed. Indeed, in this test, the high-speed/low-latency 4GB kits manage to edge out the trio of 8GB options.
Cinebench uses the Core i5 processor to render a 3D scene with over 300,000 polygons, but there's no separating the 4GB and 8GB kits. In this test, all five memory solutions are closely matched.
PCMark's overall score and GPU image manipulation tests have all five kits closely matched, but the video transcoding benchmark - which converts DV to WMV9 - shows an actual benefit in doubling up to 8GB of RAM.