GRAPHIC CARD

Clarifications

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00590356&lc=en&dlc=en&cc=us&lang=en&product=3198157

So you need to look for cards that are for the PCI Express slot, with x16 graphics.

http://www.google.com/search?q=PCI+Express+x16+graphics+cards

But generally speaking if it is a Desktop you can upgrade the Video Card by either removing the existing one and fitting a new one or if you have On Board Video fitting a Video Card tot he Available Socket. So here if you have a PCI Express Socket it should just involve fitting a suitable Video card and going from there, though you will have to install the correct Driver for you're OS to get it to work correctly.

As for the PS while 250W is on the low side it may be sufficient depending on what the remainder of the load on the system is. But if you have fitted extra Drives I would be looking at something better and particularly if you want a High End Video Card a bigger Power Supply would defiantly be a better option here. Personally I would never recommend keeping these Makers Supplied PS as they are rubbish and only rated to a Peek Load I always recommend Antec Power Supplies which are rated to Constant Draw instead of Peek.

Col

It's way to low, even low range video cards require 300watt PSU's, and that's assumine you have no extra hardware.

A 550watt is a nice size that will meet your powerneeds and just falls short the requirements for some of the hugest cards. If you wanted to run something liek a GTX280 or 295, then you need a 600watt, 1000watt respectivly.
That just gives you some numbers :)