Until you upgrade to a 64 bit OS, you won't ever see the rest of your RAM. And depending on which video card you get, you might actually lower
the available amount of RAM that your OS sees due to the limits of 32 bit operating systems. FYI.
Your CPU is also a handicap. If you consider yourself a true gamer, you'll probably want to look at getting that CPU upgraded as soon as you invest in a good video card. Fortunately you need to upgrade the video card more than the CPU at this point, so I'll focus on that.
We need to know the kind of budget you have. $100? $200? <$300?
Second, what kind of power supply is in your system? If it's not a name brand unit, don't consider getting a high end video card until it is. If it's already a name brand, we need to know what the amps & watts ratings are on all of the rails, just for completeness sake.
Welcome to the forums, btw.