Here's what I posted in a thread a few days ago to help someone out. It should also help you as well....
The first number denotes the generation, HD 4xxx, HD 5xxx, HD 6xxx. Each generation generally has more features and are more powerful than the previous generation overall.
The second number represents performance class in that particular generation, HD x4xx, HD x5xx, HD x6xxx, HD x7xx, etc. The higher the number the more powerful the card will be. Therefore, you can the following (people will have different opinions as well):
HD 54xx = multimedia card
HD 55xx = entry level / value gaming
HD 56xx = mainstream
HD 57xx = higher end mainstream
The last two numbers lets you know the difference between multiple cards in the same performance class. The larger the last two numbers, the better the performance. Therefore, the HD 5870 is faster than the HD 5850 and both are faster than the HD 5770.
Performance differences between generations (like HD 4xxx vs. the HD 5xxx series) varies. Sometime it will be a minor improvement, sometime it will be a major improvement. Going from HD 4xxx to HD 5xxx was a huge performance increase. Going from the HD 5xxx to HD 6xxx is expected to be a rather minor or moderate increase in overall performance.
How can you tell the performance difference between generations (i.e. HD 4xxx and HD 5xxxx)? Benchmarks...