clutchc :
I had 2 different HD 5770s at 2 different times. They were great at 1680x1050 but couldn't keep up the framerates I needed at 19xx X 1xx0 resolutions. But yes, the HD 5770 is superior to the HD 6770. The HD 6790 is closer to the HD 5770 today.
http://www.hwcompare.com/10021/radeon-hd-5770-vs-radeon-hd-6790/
Unfortunately, HD 5770s are getting harder to locate. You can still find them if you hunt a bit. Amazon still sells them: http://www.amazon.com/dp/B0035K6H2C/ref=asc_df_B0035K6H2C1852940?smid=ATVPDKIKX0DER&tag=hyprod-20&linkCode=asn&creative=395093&creativeASIN=B0035K6H2C
Yes, the 5770 is not made for 1080p. But boy it is a little workhorse at 1600x900 - where I ran my games in.
Crysis, I would play on high settings/AAx2 in 900p @ 60fps (overclocked slightly). For a budget card - amazing.
Unfortunately, in higher resolutions and/or higher settings the frame rate would literally crumble. But if you are willing to make a few concessions, the 5770 still offers maybe the best bang for your buck out there.
I do hear they scale terribly in crossfire, but I cannot back that up.