What is the cheapest way to most improve my cheap-ass PC? I'd like to be able to run some simple games (see comments)

Hey I heard you guys were a friendly community and I was wondering what the best and cheapest way to upgrade my PC is so that I can run some very simple games such as TF2, Portal, L4D2, amnesia, SMB, FO:NV... you get the gist - not quite top-range but decent.

At the minute most won't even open or run for long without crashing.

Here are my specs: OS: Win 7 Model: Hewlett-Packard G5330uk CPU: Pentium Dual Core E5800 (3.2 Ghz) RAM: 3GB Direct X11 GPU: Intel(R) G41

I know that it sucks but I have very little funds and just want to improve to the bare minimum (because I'm not quite there yet. I really can't iterate enough how low budget I want to keep this.

Thank you so much for any input.

Edit 1: GPU is coming up a lot - what is the cheapest unit I could get that would allow me to adequately play the mentioned games?? I doubt I need to shell out $100+ just to run a game.... right?