Are you sure of your model number of the computer? is it a123? because i coundnt find anything on google or on hp's website about it.

What you need to find out is a few things. Is it integrated, agp card, or pci express. Next you need to look at your powersupply and find out how much wattage it has (it should say on the side on a sticker.) This is important becuase most companies put small powersupplys in pc's and a new graphics card wont run because it needs more power. You may have a hard time replacing the powersupply becuase as i found out with my vaio, they use small form factor powersupplys and buying a replacement that has some power is not possible.

Post back here with some info on what you come up with and we will try to help you further.