I'd personally choose Corsair over g.Skill any day.
And yeah, if you're on a 32-bit OS make sure to keep in mind the addressing limitations of the x86 architecture; under 32-bit OS, it's simply not mathematically possible to see all 4GB of the memory. This will also depend on how much memory is being addressed by Windows by your other hardware, ie. graphics card, etc.
I am actually debating whether or not to upgrade to 4GB on my system. I feel like 2GB of RAM is bottlenecking me in WAR, but it really almost feels like a waste to buy two more gigs of RAM, only to see a fraction of it.
I think I've come to the conclusion, however, that every extra bit of memory helps.
It all comes down to what you're using your system for---I've gotten by just fine on XP for the last 2.5 years with the system I have now. WAR is the first application that's made me even consider an upgrade since I've built this rig.