Traditionally on 32 bit Windows systems with 4GB RAM installed, 2GB of RAM would be used for kernel space (OS) and 2GB for user space (application). There are some swithes to change this somewhat, but I wouldn't imagine you'd need that. Not a bad idea double your RAM for the extra $10 if you're planning to upgrade anyway.

I like the Crucial site as it will tell you exactly what type of memory you'll need for your system.

Sometimes you carpe diem, sometimes your diem gets carped.