I have a 9600GT and I'm using nTune 5.05.54 to overclock the card and it's great. I was able to get to 715mhz gpu and 1015 (or 2030 in ddr). Running benches with no problems. Never tried to get past that as it doesn't increase performance that much than a cpu overclock can.
In 3DMark05, I get 400 more points from the 9600gt overclock and 4500 more points from a cpu overclock.
In 3DMark06, I get about 200 more points from the 9600gt overclock and about 1000 more points from a cpu overclock.