|agree with Mickliq - you mention wanting this for gaming. 4GB is fine, and you should be able to run with an intel C2D for a while longer.|
You need to make sure your motherboard has a PCI-Express slot (aka PCI-e or PCI-x), but if it's running a C2D CPU, i'd bet it's got a PCIe slot. Then you just need to pick a video card that matches your needs and your budget. When you think you found a good one, start a new thread here asking for feedback before you buy.
If you've got $50, you can find some cards that would be moderate upgrades. If you're looking at spending a few hundred, you'll need to make sure your power supply can handle the added load.