I'm going to give it to you straight. There hasn't been a single Dell Dimension made that ever qualified as a gaming machine. I'm willing to state that nearly every Dell in existence is an hunk of junk that could do more for the same price.
You are still living in the days of the single-core Pentium processor, and 333mhz DDR RAM. What's worse is you have integrated graphics from before the Intel GMA series. And the GMA series is terrible right now, and those are the most up to date integrated cards there are. That should tell you something about the GMA predecessors.
No amount of upgrades could make newer(<-- keyword) games playable on a machine like that. All the spare DDR RAM in the world, and an overclocked P4 still wouldn't help you much.
Adding a new graphics card wouldn't do much for you either because of something known as bottle-necking. Basically, your graphics card can only work as fast as the other parts will allow. So if you have a blazing fast 9500gt PCI, and you've still got your DDR 333mhz and P4, you're still going to get very bad lag on most games. The other down side is that you'll need a new power supply along with a new graphics card for that machine so you'd be spending extra money regardless.
Short of buying a new computer, I don't see this as a worthwhile endeavour by any standards. Throwing money at this isn't going to help you. I suggest you stop being lazy about it and realize that this isn't going to work and that you're better off shelling out a little bit of extra money on a better computer. And even if buying a new PCI card for your old PC helps a little bit, it won't tide you over for very long at all. Perhaps a few months at best.
What you need is a PC that supports most current and perhaps some of the new processors, graphics cards, and memory that are becoming available. It needs to be upgradeable so that in the future, if something new comes out, you can still purchase it and use it with your PC. Something that has been planned in accordance with the future is your best bet.
It has to support current Intel Dual and Quad Cores or AMD Phenom cores, it has to support DDR3 memory, it has to support PCI-Express 2.0, and it has to have at least 450 or 600 watts of PSU power. If you want to play newer games, those are the minimums for good framerates these days.
If you'd rather stick with older games and just get your computer to the point where it can play them without serious effort, then fine. You can stick with the Dimension and add something perhaps. But if you start feeling the desire to play newer games, then I've made my point.
to game with !
