Here you have a nice review of a HD 5770 of MSI that comes overclocked from 850mhz to 875mhz. The charts show an 2-4 fps increase overall to a stock 5770.
http://www.guru3d.com/article/msi-r5770-hawk-review/11
Yet at 900mhz, I don't think it will surpass the 4890 on most cases(at dx11 it may surpass it alright), but overclocked will show a difference of -2/5 fps vs the 4890.
If you take on account the new tech of the 5xxx cards, 2/5 fps less than 4890 is not important at all. I would go with the 5770 without thinking twice(you can also look for 5770 OC'ed card versions(sapphire, asus, MSI they have a nice overclocked cards with custom coolers) ).
Note that if you buy an OC'ed version of 5770, when you decide to crossfire it, you will need to buy the same card as you did(not obligatory but recommended), also OC'ed versions with custome coolers, will work cooler, witch in the future, if you wanted to do a unoficial overclock, you will have a lot of heatroom to work with.