Which model of HP do you have? Also, what is your planned budget?
The model is needed to determine the type of slots (AGP, PCIe, or PCI) that you have available and the wattage of your power supply. At a minimum, we need this info to provide a recommendation.
HP Pavilion Slimline s3600f PC is the model,and as I said I am not loking for anything fancy Im not a gamer I just want to be able to use my computer for work.