|PCI-E x16 replaced AGP 8X as the upgrade slot for graphics cards about 4-5 years ago. There are plenty of x16 cards to choose from but to be able to help, we need to know the purpose...HD video? gaming? If decent gaming is what you're after, we'd need an idea of which games...some require more 'horsepower' than others.|
I'm sure you have a power supply with a 24-pin main plug with 4-pin ATX12V plug, but kimsland is correct in that you should open your case & get the specs off the power supply label. Look for a brand name, model number, total wattage rating and the amperage ratings for the +3.3v, +5v & +12v rails. Of these, the +12v is the most important. If your power supply is borderline, it will be up to you whether to upgrade it along with the card or to just install a low power card. Anything you install will be better than the crappy Intel GMA950 integrated graphics.
The Radeon HD4670 is the card most often recommended for system with weak power supplies. It has very good performance & uses very little power compared to other gaming cards. Have a look at the following article:
Best Graphics Cards For The Money: December '09