To the folks who suggested Kalroth's cgminer with the xintensity feature, you have my thanks. His version is better in pretty much every way, near as I can tell. I have increased hash rates across the board. And I can run my Linux desktop on top of a 7950 doing 600kh/s without it seeming lagged.
Incidentally, I have my 7950's running with xintensity 4, gpu-threads 2, and shaders 1792. 280x's the same except shaders 2048.
Oh, and with this version, I can underclock the memory on my 7950's all the way down to 900Mhz with no change in hash rate, which saves a significant amount of power according to my kill-a-watt. The 280x's wouldn't accept anything but the default memory clock without throwing a fit, however.
Wait a minute, you are getting 600KH on a 7950 @stock? Can you please post all your settings and setup? I am barely getting 550KH @1050/1500. Thanks!
Using Kalroth's cgminer, I'm getting 620 to 650 on both XFX Double D 7950's and Sapphire dual-x 7950's, depending on how finicky a specific card is. 630 seems to be average with stock voltage, core clock 1055, and memory clock at stock (sapphire) or 900Mhz (XFX).
"xintensity" : "4",
"gpu-powertune" : "20",
"gpu-threads" : "2",
"shaders" : "1792",
"gpu-fan" : "0-100",
"auto-fan" : true,
"temp-target" : "70",
Those last 3 lines are important, as the sapphire cards love to auto-throttle if they get too warm. And my 280x's are even worse.