I used to have that very computer. I upgraded everything in it except the motherboard. I installed the fastest CPU, the works.
If you want a new GPU (video card) you'll need a new PSU (power supply) Then you can buy something on ebay like a GT 8800. I have one actually. I'd have to dig around and find it. But the critical component is that you need a bigger PSU that puts out more wattage.
What kind of games do you play? If you intend on playing current games then there is no way. You could play Battlefield 2. I use the AIX mod. You can get the BF2 client to play at BF2Hub. I even played COD Modern warfare on that PC. Even F.E.A.R. You are limited to games up to the year 2008 or thereabouts, and even then you may have to turn down some graphics settings.
You're not going to have much of a gaming PC for 5 bucks I'm afraid. If you want a half way decent PC, consider a budget of at least $600-$800. That'll get you into the more modern games.