I can't really help you all that much, but I had 2gb of GEiL PC6400C4 and I bought another 2gb; the difference was astronomical all across the board, be opening WMP11 first thing when I turn the computer on, playing on World of Warcraft or working on apps such as 3dsmax. Using Vista x64 Home Premium here and a Core 2 Duo 2.54ghz.
So basically, huge noticeable difference from 2 to 4gb.