My 7950 Windforce 3GB OCs manage 610-630 kh/s each and I don't overclock or undervolt.
I also use Gigabyte cards.
7870 OC latest rev
7950 WindForce revision 2. (windforce is the non OC version - same fan as the OC, yet comes at a lower clock)
7950 Windforce Revision 2 from Gigabyte do not come on reference 7950 PCBs though, they come on a 7970PCB.
Mine all happy overclock - but I do underclock the ram to reduce temperature and save around 20watts per card down to 300 or 310mhz.
My 7870s pull up to 550mhash (I mine bitcoins) and my 7950s gather up to around 650mhs.
However as I like to run at around 60*C, I have them auto tune and they run at 400 and 550mhash respectively.