If you want a GAMING computer, you're going to need to do two things:
1) Raise your budget to at least $450
2) Recycle or re-use SOMETHING from your existing system.
Keyboard, mouse, monitor and OS would be the barebones minimum you would want to re-use with your new system in order to save money.
At this stage, you could build a very decent gaming computer that can play many games in low to medium settings well, plus be expandable in the future for more complex/taxing games.
I can definately put you a parts list together for a good, AM2 system that is fully future upgradable and will play Source engine games decently for around $350-$400 + tax and shipping.. but it would be without an Operating System, monitor, keyboard, mouse, etc.etc.