I'm using EVGA precision to overclock my Zotac 9800GT. When I run the benchmark in GTA4 I actually lose about .5 fps when I bump the core clock from 660 to 700. Any idea why this might happen? I have a new 500 watt power supply and the GPU temp stays well below 70 degrees.
I've experienced this a few times myself with other cards. That's why its always wise to OC in increments and then test performance it even happens with CPU's so try going down to 670 and bench from there until you se a drop in performance.