Based on all available information Not much of a diff between 4890 & 5850, except in some games. GTX295 is better in some vs 5870. Power needed is all most the same between 4890/5770/5750/5870/5850.
Therefore the Price between these cards will be the factor to buy.
Right!

Thats why we always go at Sri Lankan sales reps

Its the idle power and the processing power.
Here I take 4890 vs 5870.And Itake the latest reviewed HD 5770 charts from techpowerup.com.Reason: W1zzard , who makes GPU-Z,Radeon BIOS Editor and the maker of ATI Tool and the admin of techpowerup.com,is the reviewer.I don't think anybody knows as much as him of these other than ppl at AMD.
A good clear chart from techpowerup
Mostly a PC would be in Idle.A 42W power save on the idle
HD 5850 has Approx 37% more power than HD 4890 in 1680 x 1050.Note that with this kind of VGA don't compare 1024 x 768

People buy these obviously has better displays and games tend to cap the frame limit cuz there will be unneeded processing in low resolutions.
Added DX 11(Which is backwards compatible with DX9,10,10.1 unlike DX 10.1), 6 DACs which will support any combination of 6 display outputs,Native HDMI with Native 7.1 audio compliant with HDMI 1.3a (Yes , I use HDMI with Dolby in our 32" TV ),low power consumption, Ill pay that extra $50 for it.And you will be able to take advantage in DX 11 next year as there are many shifting to DX11 as it has backwords compatibility for 9.0c.You can see videos in youtube coders saying that they can turn 10 engines into 11 in 1 hour.I'm not sure about 1 hour though but it is happening.