It's confusing but yeah.Each series (5XXX, 6XXX etc) has a lineup. Each card in the lineup is numbered in order from best to worse (worst being say 6310, best 6990), however the numbering system doesn't carry over between series'.But it can be used as a general guideline that say a 5670 isn't as good as a 6670. There are a few exceptions though.
Like how the HD 5770 is mostly the same as the HD 6800 series
