How important is DX 11?
How do the HD5770 compare to a GeForce GTS 250 or 260 and how about using two older cards?
Depends on what games your play. For a midrange card DX11 is not that important as in my own opinion it doesn't have enough power to run DX11 graphics well enough at higher settings. A higher ends card will benefit from it as it can handle the increased graphics load. It is not that important right now, but once games start coming out that support DX11, they would(should) look better in DX11 mode.
The 5770 performs pretty much between the GTS250 and GTX260. Being that it outperforms the 250, but is outperformed by the 260.
See Tom's article on the 5770: http://www.tomshardware.com/reviews/radeon-hd-5770,2446.html