It is possible to get high hash rates by OC'ing the eyeballs off your 7950, but the higher you push them the shorter the period they will stay up.  One thing I noticed about all of my 7950's is that don't perform well with low intensity settings "-I 13" for example... like a 7970 will.  So you have to go for the higher "I" settings... personally I can run a 7950 for weeks if I keep the hash rate below 600kh/s.  I try to keep mine at 575kh/s average.  Since my rigs are 20 miles from my house...its more important for me to keep them running around the clock... so I'm willing to sacrifice a bit sprinting...and treat it more like a marathon...