It's actually half correct to say the 6 series 'replaces' the 5 series, the 6 Series technically fills the gaps between the cards within the 5 series range.
Like been displayed on the post above me...the y-axis (saturn85).
But IMO i think the 6850 truly
replaced the 5830 since market wise the 5830 seems too expensive for it's performance, therefore had to be booted out, plus paying the cheaper price for 6850 for better/equal performance to the more expensive 5830 is more favourable to the consumers.
I like the 6870 bridging the gap between 5850 and 5870, plus you can easily OC the 6870 to equalize the 5870's performance.
The addition of 6950 and 6970 are logical, since the 5970's gap with 5870 is rather huge, therefore need two cards to bridge the gap; and it's more flexible for consumer's this way.
The 6990, is just to piss off Nvidia I guess