Haha, I still have a better computer other than monitor.
Unfortunately your power supply is 250 watts, which is weak for gaming IMO. The most you can probably get is a GeForce in the 7000s, or if your lucky, the 8400s. Most of the things below the 8 series though can't support DirectX 10, which will cause problems in Vista gaming.
Pretty much, you went ahead and bought a crappy computer and got ripped off. If you haven't bought it yet, I suggest you don't.
Also, yes, what's your budget?