Dicho,
Open the case and look at the power supply. The box HP generally has the minimum power supply for the way it is equipped out the door. If you do not have at least a 450 power supply you will need a larger one so add another $50 to $100 to the cost depending on what power supply you buy. I bought an HP about 3 years ago now and wanted to add a vid card to it as the built in one just didn't do the games that well. The power supply issue was a road block for me. I need to get that first before I buy even a 2 year old card. Don't need cutting edge for box games but better than 500 megs built in sharing the board memory for sure.