700mhz is around the typical limit for 8800gt's lower binned g92 core with 112 sp, although you can certainly try higher. Some people have gotten 750mhz before. For 8800gts/9800gtx's g92 with full 128 sp, the limit is 750-800mhz.
For temperature, it's fine as long as it's below 85C. Use this for stress test, stability mode, default resolution, no aa.http://www.ozone3d.net/benchmarks/fur/