Could someone please helpout a guy that knows just enough about PCs to know he is in over his head?

I have a Dell Dimension 9100 desktop I purchased back in '05. I am looking to upgrade the video card in it. I would like to max out the PC's Video capabilities for gaming without throwing away money on a card that would be bottlenecked by the other parts of my PC.

Here are the basics of the system...

Pentium 4 Prescott DT 3.0GHz
1.0 Gb DIMM 512
Flat Screen Monitor, 1280x1024 natural resolution

I was looking at the new ATI 3850 or 3870 with 512Mb... but not sure if that makes sense with the older CPU.

Certainly will change power supply and add memory if needed.

Any help would be greatly appreciated! Feel free to post or PM.