Can we get your exact system specs? With those prebuilt systems its really hard to recommend cards as we don't know what your mobo (Motherboard) supports and what CPU you have (and thus how far you can go).
I can say with a 230 Watt Stock HP PSU you are not going to be able to upgrade. You will want to upgrade the PSU before you try a graphics upgrade or you won't have enough juice.
i read the specs. it says pci express slots 1........ but it also says occupied. occupied by what ?
and if it's unoccupied i think the biggest card you could put in it the way it is is a 5670..... requires no external power connection and should work with the small power supply/
Its occupied by a Geforce 6200 apparently from the specs. Its basically an Integrated chip with its own memory... Better than the integrated from its time but not buy much and also much slower than todays integrated solutions....
The Pentium 4 is VERY slow for today's standards (Good little chip in its day though). I wouldn't try anything higher than even an ATI 5670. I would go with at least 400 watts to be safe with the 5670 (or the nVidia GeForce GT 520 mentioned below). Though you would be fine with something around 300-350, I like a lot of breathing room. Make sure you get a good PSU make or you might have serious problems later on (ie Component(s) failure):
For the GPU this is the last single slotter ATI 5670 model that Newegg is still selling:
Remember that both of these guys are really going to be bottlenecked by the P4. The last recommendation, which actually should have been at the beginning was the nVidia GeForce GT 520. Here is the entire list of them from Newegg (Always recommend the EVGA and MSI models as they have the better support):