Here' s there thing,I got a piece of shit computer for $400 and now I wanna start putting some power into it. Problem is I dont know where to start and I have a low budget.

Here' s the current specs or at least what i know about it. Hell I mso computer illiterate Im not even sure what counts as specs.

512mb Memory
AMD 64 Athlon 3500 Processor
160gb Hard Drive
Nvidia GeForce 6150 LE
The OS is Windows XP media Center addition.

Im not looking for playing the newest of games in the highest of quality or anything. Just pretty basic stuff really.

Games that I currently have but cant really run to well include:
Vampire Masquarade:Bloodlines
Garry' s Mod (which required getting a source powered game so I picked up Counterstrike)
The Movies

The first one I cant run for shit. the 2nd runs well enough but does take time to load up and its a bit jumpy every so often. The Movies well enough on medium settings. I would like to run Oblivion just so I can mod the fuck out of it since that seems to be all I end up doing with a PC game anyhow. But Im guessing It aint gonna be easy to achive that huh? But like I said I dont need high end visuals.

So where should I start?