I want to share my settings for HD5870 and x11mod (used sph-sgminer_x11mod_b2_2min_SICK.zip):

Windows 7 32bit, Catalyst 14.4
2xHD5870 Powercolor PCS+

850/975 1040mV    ~1755 KHash/s per card (I'm using this)
875/1225 (default) ~1855 KHash/s per card
920/1200              ~1918 KHash/s per card
950/1200              ~1967 KHash/s per card

"intensity" : "19,19",
"worksize" : "128,128",
"kernel" : "x11mod",
"lookup-gap" : "2,2",
"thread-concurrency" : "8000,8000",
"gpu-threads" : "1,1",

http://i.imgur.com/IxzkLCn.png