First you need to look at the several factors surrounding the question 'what do you want in a graphics card?' Do you want the best quality, the best price offer, dual screen monitor support, what kind of games are you going play and so on. If you are into first person shooters at the highest quality you might spend around 300 to 500 dollars, but if you love strategy games then you might only spend under a $ 100, it makes a big difference.
So as you research, what price range do you have in mind? Are you willing to spend over 100 dollars for a high quality graphics card or do you want to stick with a basic model? Also, if your wondering whether the card in your computer is good enough to play the games you want?
The answer is most of the time no. The one put inside computer (except it is a gaming computer) are usually bare minimum and not optimized for high computer gaming. And even if your video settings read 128MB in your system, it is not always that, because your graphics card can be used shared ram. A long story short if your wanting to play any of the bigger RPG's or First person shooters then your gonna need a new one. On the other hand if your playing MMOs or (Massive Multiplayer Online games) or older RTS or (real time strategy) then you might do just fine.
On to bigger, better things, you need to look at how much MB of memory the card has and whether you need an AGP or PCI graphics card. In order to find out whether your slot is an AGP or PCI Click Here , it will give you the step by step on how to figure this out. In order to find out how much MB or memory you want its really up to you. It ranges from about 128MB to 768MB depending on how great you want your PC games to run. Most of the better games require a better one that will perform later well, and requires a higher memory. Other than that you should not really have to check much else, the card you buy (if fairly new) should come with all the newest shaders and parts tact on to the unit.