Some of them respond well to voltmodding and some don't. My 9600GSO (Newegg calls it "EVGA 384-P3-N966-TR GeForce 9600 GSO 384MB 192-bit GDDR3", the one with the double-height rear exhaust aluminum heatsink), was voltmodded with the hope of better performance but it barely clocked any higher and heat wasn't yet the issue. Due to this I accepted the ~ 5% lower o'c, keeping in mind these are chips nVidia has the underfill and solder bump problem on so they should last longer if not pushed to their short-term limits. With and without the voltmod it initially seemed like the card could do about 720 core, 1060 memory, but after testing with ATI Tool for a few hours it turned out that the card couldn't stay 100% stable till backed down to 640 core, 960 mem. at full fan speed. It might've been stable at 660/980, I usually subtract 20 to give a little margin then with that confidence level I can go ahead and flash a modded bios to make it permanent.
Funny thing is, backing the fan speed down to 40% didn't cause a significant rise in temperature, only a single-digit # of degrees. Next I took the 'sink off, lapped the area over the core and put AS grease on. That gained 3', not enough to have bothered with especially when the memory seems the bottleneck but it wasn't even getting warm enough to bother with heatsinking.