How to tell how good a Radeon card is by name
So how do you read a Radeon?
Somewhere on the card, it'll say it's product number. Compare it to other cards on this link: http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32374-7.html
AMD's naming convention is the same as nVidia; but the model name use 4 numbers instead of 3. Let's use the Radeon HD 6950 as an example....
HD 6xxx = The "6" represents the generation.
HD 69xx = The "9" represents how powerful the card is; meaning performance level.
HD 6950 = The "50" represents how powerful that card is compared to other cards that could be in that same performance level. The HD 6950 is faster than the HD 6930, but slower than both the HD 6970 and HD 6990 (highest end card with dual GPU cores).
HD 69xx = Premium performance
HD 68xx = High End performance
HD 67xx = Mid Level performance
HD 66xx = Mainstream performance
HD 65xx = Entry Level performance
HD 64xx = Basic performance
To be a little more detailed on the comparison...
The "hundreds" digit tends to denote the GPU core type used, or, if more than two different segments (often, say, the x6xx and x5xx cards) use the same GPU, some major fundamental difference between the two.
Lastly, the "tens" digit denotes a small difference between models. Again, like with the second digit, the higher number denotes a directly superior card; just that the difference is smaller.
Also, much to AMD's shame, they've shifted things a little over time: originally, for the 4xxx and 5xxx cards, there were no single-GPU x9xx cards; there were no 49xx cards at all, while the 59xx were exclusively dual-GPU. For the more current 69xx and 79xx cards, though, the pattern's far more consistent:
x990 - Top card, dual-GPU, all cores enabled.
x970 - Top single-GPU card, all cores enabled.
x950 - Same GPU as above, but some cores disabled/clock lowered
x870 - Second-tier GPU, with all of its cores enabled. (still below the x950)
x850 - Second-tier GPU, with some cores disabled.
As for the lower-end cards, jaguarskx was a bit more euphemistic:
900 series - Top GPU.
800 series - Second-tier GPU. Still high-end performance, often the best "bang for your buck."
700 series - Midrange GPU, still useable gaming performance, but not that potent.
600 series - Upper-entry-level. Often similar, just slightly better than the 500s.
500 series - Lower-entry-level GPU. Similar to the 600s, and still may be worthwhile as a gaming card for a rig on a particularly tight budget.
400/300 series - Rock-bottom, made for OEM machines. Never buy; they're vastly below even the 500s.
200/100 series - In case you were curious, these two sections today are reserved entirely for naming integrated graphics chips.