try adjusting the intensity to 9,9 (under GPU menu)
I have a similar setup with two 7970s in one rig (without crossfire bridge connecting them). Running Win7 x64 and BFGMiner with just the -I 9 flag, and am getting about 650 Mh/s per card with core clocks set to 1075 MHz.
Set intensity to 9 with the same results. Temps are lower because I've left it off:
cgminer version 2.11.4 - Started: [2013-04-18 19:00:02]
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(5s):660.8M (avg):652.2Mh/s | A:9 R:0 HW:0 U:8.1/m WU:8.1/m
ST: 2 SS: 0 NB: 1 LW: 24 GF: 0 RF: 0
Connected to stratum.btcguild.com diff 1 with stratum as user user_miner
Block: 01a5b95f64f506aa... Diff:8.97M Started: [19:00:02] Best share: 81
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[P]ool management [G]PU management [S]ettings [D]isplay options [Q]uit
GPU 0: 74.0C 3637RPM | 328.9M/365.4Mh/s | A:4 R:0 HW:0 U: 3.62/m I: 9
GPU 1: 48.0C 3425RPM | 328.8M/324.8Mh/s | A:5 R:0 HW:0 U: 4.52/m I: 9
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Any more thoughts?