Video cards are packaged in two main ways, the firsts is "onboard" in which the video processing unit is manufactured into the systems motherboard. The second type of video processor is an "add in card", add in cards are available in 3 main types: AGP, PCI, PCI-Express. PCI add in cards are the slowest as PCI slots have the lowest overall bandwidth to the system and should be avoided if AGP or PCI-E is available. AGP Stand for accelerated graphic port, as you may well assume These ports were intended specifically to handle the high bandwidth that video card consume. PCI-Express is the newest type of port available to video cards and likewise has the highest data bandwidth of all the preceding port, making it the best choice for modern video cards if available.
There are two major chip manufacturers that produce Graphics Processing Unit (GPU) that are primarily available today. ATI, now a divison of AMD, and Nvidia produce most of the chips that power these video cards and yet other manufacterer buy the processors and produce the brand names that are associated with the different video cards. Though many of these different video cards may carry the same processor performance may vary quite a bit. Choose carefully.
Nvida and ATI both have thier own strengths and weaknesses as do the different manufactures of the cards, the main thing to keep in mind are the features you want and the price you wish to pay. Each chip maker categorizes its processors into categories based on features, and how fast they perform. Each manufacturer posts these categories on the package of their video card. Nvidia's GPU's are categorizes as four digit numbers the first representing the feature set, and the next three representing relative performance. For example Nvida Geforce 8600, the 8 represents that its an 8th generation processor all of which support the new Direct-x 10 Feature set, and the 600 shows that the speed will be faster than the geforce 8500 and slower than the 8900. ATI uses a similar system however they recently switched thier lable systems which can cause confusion. All of ATI's newer video card begin with an "X" the older cards although they have Higher numbers do not have the "X" and represent older and slower technologies. The first example is of the old system. Radeon 9550: the 9 represents the line of features the video card has and the 550 is a relative measure of the speed. meaning Radeon 9550 will carry more features than the radeon 8000, but will not be as fast as the radeon 9600. The new system is the same however as ATI reached the number 10000 they decided to start over from 100 adding an X on the front. The Radeon X1250 will carry more features than the Radeon x500, or the Radeon 9250, but be slower than the x1500.