How accurate is the hierarchy ?

you just happen to compare the weakest card in a tier to the strongest, theres going to be a decent difference but generally the newer cards are quiet a bit faster than the old card in the same tier.

the tiers just gives a ruff picture, you need to look at real performance to really see the differences.

^+1
Also check out the bench marks to get a down and personal look for what games they work on the best and you can use the compairson charts also to help sort out the little stuff.

The 8800 Ultra was the fastest card made when it was released. The GTS 450 and HD 6750 were lower midrange budget cards when new.

The 8800GTX is actually around 1-3% faster then the 9800GTX+. All of those cards are probably within ~5% of each other. The big advantage you get with the newer cards is they use less power, and better/more features like DX11, etc.

Like what everybody else said They are using the benchmarks for the Hierarchy to give you some what of an idea as to how the card fall or lay out. I would use that chart to help me to decide a card.