This is GENERALLY how to classify how "good" or "fast" a graphics card is:
Split them up into two categories (as mentioned above): Nvidia Geforce and ATI (owned by AMD) Radeon.
From there, look at the number. It should be three or four digits, plus some letters and other junk. Examples are:
Radeon 9800 Pro (NOT to be confused with a Geforce 9800 GT / GTX / GTX+)
Radeon X1650 Pro
Radeon X1950 XTX
Radeon HD 3850
Radeon HD 4350
Radeon HD 4670
Radeon HD 4850
Radeon HD 4890
Radeon HD 5770
Radeon HD 5870
In general, the first digit of the number is the series of the card. So, in this list, we see a 9000 series card, two X1000 series cards, an HD 3000 series card, four HD 4000 series cards, and two HD 5000 series cards. USUALLY the higher the number, the newer the card is, but we see an exception here: the 9000 series is the highest number, but it's actually the oldest card in the list. When in doubt, do a quick Google search for the card's name and see what results you get. If you get reviews from 2-3 years ago, the card is probably too old to consider.
The other numbers in each card's name denotes it's place within the series. Once again, the higher the better. So, if we look at the HD 4000 series cards, we have a 4350, 4670, 4850, and 4890. The HD 4350 is the slowest, and the HD 4890 is the fastest. However, the speed of each card is NOT directly proportionate to its number - for example, we can't take an HD 4350's abysmal performance, then double the number (making it an HD 4700, which doesn't exist) and from that assume that an HD 4770 (which does exist) should be a little over twice as fast. The HD 4770 is actually many times faster than the HD 4350. So remember, they're just naming schemes, not something to take at face value.
Often times you'll see a lesser card for sale at Best Buy (or similar places) with all kinds of flashy stickers and cool pictures that will just make it seem like it should tear through any games you throw at it. You'll be horribly disappointed when you bring your new HD 4650 home (which you spent $90 on) and lo and behold, it can't play Call of Duty: Modern Warfare 2 at the highest settings. For the most part, you can guess what a card is intended for based on how high or low it appears in it's series. Let's use the Radeon HD 4000 series again. A card low in a given series (4000 - 4400 or so in this example) is usually intended for 2D desktop and possible home theater use (NOT gaming). A card in the mid range (4500 - 4600) is good for home theater and light to medium gaming, but won't handle any cutting-edge games. A card high in the series (4700-4900) is almost always built purposely for gaming and should deliver a good gaming experience.
One thing that trips many people up is the jump from series to series. If an HD 3850 was fast for it's time, an HD 4350 should be even faster, right? I mean, it's in the four thousands, and the other card is only in the three thousands! Wrong. Again, generally speaking, the lower the card is in it's respective series, the lower the performance and the less things it's intended to do. An HD 4350 is not a card meant for gaming, while an HD 3850 was. So even though the 4350 is a newer card and has a higher number overall, it's place so low in it's series makes it a sub-par performing card, and the HD 3850 will blow it away if you want to play some games.
Even when you have a specific card model selected (for example, an HD 4670), there may be differences from card to card and brand to brand. Often times (especially at the mid-range), the same model will come with varying amounts and speeds of memory. How much graphics memory you need largely depends on your monitor's resolution and what games you play, but in general faster memory is better, and more memory is better. A smaller amount of fast memory (ex: 512 MB of DDR5) will usually give you better performance than a larger amount of slow memory (ex: 1 GB of DDR3).
There are several other things to consider. Gaming cards these days consume a lot of power, and will often need a decent power supply with the right (PCI-E) connector(s). Make sure your power supply is up to the task. You'll also need to make sure your motherboard supports the type of card you're about to buy. If you have an older motherboard with an AGP slot (or only PCI slots), you won't be able to use a PCI Express (PCI-E) graphics card. Finally, make sure the card will physically fit in your case (some newer gaming cards are very long), and it'll have the right video outputs for your setup (VGA, DVI, HDMI, DisplayPort, etc.).
Hopefully this has helped to give you a general sense on how to shop for graphics cards, and hasn't just added to the confusion.
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