As I said it's just my opinion. For standard gaming your onboard graphics are pretty good. For adanced gaming you'll need more than this computer or a card can supply. Then there is the cost factor.
Look at your specs here. http://h10025.www1.hp.com/ewfrf/wc/documentSubCategory?tmp_rule=42878&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&product=3801314&key=null&site=null
Costs are the card and much bigger PSU. I would include the cost of more ram but yours will only support 4Gb and all of that will not be used on your 32 bit system.
For a good gaming computer a quad core, 16Gb ram, graphics card, bigger PSU to support it etc. I would thing it better to buy a new and better PC rather than do a partial upgrade at that expense.