I just know how ATI is setting their numbers for years now
example
6990 =
first digit = generation , 6000 series is generation of 2010 so 5000 series is 2009
second digit = rank. the higher the number is , close to 9 , the better it is.
3=ultra cheap
5 = medium
7=pro
9=xtreme
the third digit is the level of the card similiar to 2nd digit the higher the number is , close to 9 , the better it is.
so it goes like the above
3=ultra cheap
5 = medium
7=pro
9=xtreme
So this year (2011) AMD is gonna release the next generation (7xxx series) and the best is gonna be the 7990.