G.Skill RAM is good, but the money's better spent on GPU/CPU. Besides, the things that are nice about G.Skill only really come out when you start overclocking. Otherwise, stock RAM works almost just as well. You just need to make sure that your RAM frequency is paired nicely with the CPU / FSB speed (or the intended speed, if you want to overclock). I forget the exact formula, but you might find it in one of my posts a while ago (something like FSB x number of channels = ideal memory frequency).
Most people are starting to jump ship over to 64-bit...just install a 64-bit OS as it has become well supported from what I understand.
RE: 3GB vs 4GB. The reason why you only get a piece of the 4GB in a 32-bit environment is that other memory needs to be addressed as well - e.g., video RAM. There are workarounds for this, though, and they do exist in Windows IMHO.
Quad-cores are still of limited utility - it's mostly applications such as Photoshop, Premiere, CAD and 3D modelling software that are multithreaded. Some games (slowly) are, but you're better of getting a higher performance dual core. Besides, C2Ds usually have more headroom for OC if you go down that path.