To start with, is there an overclocked version of your card? If there is use those clocks. My card came stock at 650/900 but the overclocked version was 700/1000 so I started there. Now, slowly work up. Use ATiTool to make sure there are no artifacts or anything. I don't think many 9600GTs can go higher than 750 on the core but who knows.
Every card is different. What your friend got might not reflect the overclock you achieved. Can you link the exact model of 9600GT you have please?
This is getting annoying, did I get a bad batch or something? I can't overclock this card at all. A friend of mine thinks its artifacts, but I can't see how such a mild OC can cause the screen to flicker and after abou 5 seconds lock up. I don't know, i'm still new to overlclocking, so that may as well be the cause. =
GPU temp is 38 idle and 39 with OC.