2Gb -> 4Gb actually slows down processor intensive applications in Vista 64, the proper improvement is from 2Gb -> 8Gb or more.
Hehe

I wonder how much of that is from memory leaks and bloat code in libraries?
Leopard has the same issues. With 10.5.3 (next update) there's several memory leaks fixed.. I was hitting 3.2GB during Xcode compiles yesterday with a general average around 2GB used...
Also the more memory channels available (slots used) to the memory controller the bigger the bandwidth for quad core chips.