How do I sort out the specs?
The first two things that any manufacturer will tell you about a card (well, other than the fact that they made it) are the type of graphics chip involved and the amount of memory onboard. Thankfully, these really are the two most important facts to know. First, a quick look at the graphics processing units (GPUs).
Graphics processing units
ATI and Nvidia are the two major manufacturers of graphics processors today. Other manufacturers build actual cards around these processors, and decide things like what sort of graphic ports are on the card, and so on. Both of the chip manufacturers utilize part numbers that give a rough guide as to which "generation" or line they belong to, and how powerful it is within that line. A part number is four digits, with each number signifying specific information about the card. The first (thousands place) is the general line the chip is in; the second (hundreds place) is a good guide to the relative power (lower numbers--generally, 0-2--are low-power budget parts, while higher numbers--8 or 9--are for performance systems, with the numbers 4 to 7 marking various grades of mid-range mainstream processors).
When a new line of processors is introduced, the high-performance parts are generally released first, with midrange and budget versions following some months later, along with some tweaks to the performance products, as ATI and Nvidia try to top each other. Describing just where anything falls in this range is difficult, especially as there are so many variations coming out currently (with incremental numbers in the last two digits, and various suffixes to try and describe the GPU) that it is hard for reviewers to keep up with them.
ATI's consumer grade cards are called Radeon. The last DirectX 9 series had names such as X1650, and can be identified by the "X1" portion of the name. ATI's initial DirectX 10 series was the HD 2000, which was quickly updated with the similar HD 3000 series. Its current products are the HD 4000 series, which have changed focus from the ultra-high-end enthusiast market to the "performance" market.
Nvidia's GeForce line is intended for the general consumer, and is the competitor to the Radeon. The final DirectX 9 version was the GeForce 7000-series, with the 8000 series being the first DirectX 10 cards out the gate and the 9000 series coming out in early 2008. The current cards changed the numbering scheme to a three-digit pattern, with the GTX 200 series, which was introduced in June 2008.
But which card is best?
Of course, the immediate question anyone will come up with given a choice between two companies like this is, "Which one is better?" Overall, the answer is neither. Each one has had its successes and failures, and debates about which particular card is currently the top performer, or which one gives the best performance for the money, is a subject for benchmark-laden product reviews. Rest assured that the competition in the industry is so cutthroat that the only way for any company to stay in business is to continue offering good performance for the money. The best is a constantly moving target that would render this guide obsolete in weeks, and if you have a need for that kind of power, please do research on the current cream of the crop.
Memory is one of those things that you can never have too much of. This is just as true for video memory as for main system RAM. Current GPU designs have architectures that specify the amount of RAM that is to be used with them, which means that choices are now more limited. However, the amount of RAM provided can still be a good indicator of what market segment a graphics card is aimed at, as budget versions will have trimmed buses and therefore require less RAM.