There was another HP in a box for 100 bucks less that had a card (some low end ATI HD 3xxx) and quad core, but only 4 gig mem... I got dual core, 8gig mem, onboard Nvidia 9100... those were the only differences...... but mine was 100 bucks more!? why?
Luckily the motherboard does have a PCI-e x16 slot so you can add in a video card.
With that 300W power supply you should be able to use a Radeon HD 4670 or GeForce 9600 GSO or 9600GT. Best Graphics Cards For The Money: November '09
If you don't mind pushing your PSU a little bit harder you could step up to the GeForce 9800 GT or Radeon HD 4830.