The main problem with an old case like that would be supplies now are atx while those old prebuilds were mini tower AT type cases. Generally you always had to go back to HP, Dell, Gateway, or whoever to get a replacement supply. 200w is certainly "old" technology since prebuilds have generally seen 300w supplies now for some time now once the 478 P4s and Socket 754s and 939s were out with the new 12v cpu feed added first with the P4 boards.
The newer power distribution on the later model boards saw the need for a higher minimum requirement. If you were working with a slightly newer model seeing a 300 or 301w supply HP would likely have a 350w supply as an option.
At least when you can get into a custom build you decide there what goes in as well as how large the supply is for what you need. With a good "reliable" supply seeing adequate power you can choose just what card you want in.
It's an investment but can make you very happy in the long run. A basic case even bare bones will tend to climb quickly over the $300 mark however. A good case with good air flow generally sees about $70 depending on style and type for a quality case. A good board can be seen under $90 at times with the prices on DDR2 dropping fast lately. Start with a good board first and work up from there.