Wanting to upgrade an old computer

Alright. Here's the deal. I have a Dell Dimension 4550 desktop at this moment. I want to upgrade this, on the theory that upgrading the old system will give me a workable (if not spectacular) system. My total budget is preferably around 300 dollars, though I imagine that I could go over that if needed.

http://support.dell.com/support/ed [...] &cs=&s=gen

That has a lot of the system specs. I have a Radeon 9500 Pro video card in it at present, and the rest is pretty much stock-it's a 2.4 GHz processor, I believe, with nothing else spectacular. It came with 256 MB of DDR SDRAM, and I've added a 512 stick as well, for a total of 700-something.

My current possible intentions:
I want to upgrade the RAM-if I can get it at a good price I'll replace one or both of the ram modules with 512, for 1 gig of ram. I think I read someplace that this particular model of computer doesn't support dual-channel ram, anyway, so...

I want to upgrade the video card. It is, as the link will state, a 4x AGP slot, and I want the best I can get within reason. The current power supply is 250 watts-I can try replacing it if absolutely necessary.

Bear in mind that I don't need a high-end gaming rig here, I just want to be able to run some newer games. I'd like to be able to run Oblivion or Fallout 3, for example. Lot of newer games (for me) that I'd like to be able to run.