You will receive about 700MHash/s if you overclock them a little bit.
My 2x5850 (960/300) make about 700 stable and 750 at peak.

about buying hardware:
just calculate which card would first earn money for itselt
that's why im my local market (Poland) buying 5850 is most profitable purchase
5850: 350 Hhash/s / 200 $ = 1,75 Mhash/s/$
5870: 420 Hhash/s / 327 $ = 1,28 Mhash/s/$