Yup it's just double your memory clock. 900x2=1800. I don't have the exact facts about it though. As for temps, I heard those 8800gtx get pretty hot but I think average temps are around 50-60C. So keep your temps around that range or lower. Have fun overclocking that GTX
I use rivatuner to monitor gpu temps also. The actual gpu temps should be labeled "Core Temperature 0" and "Core Temperature 1" that is displayed on the "Hardware Monitoring".
Main -> Customize -> Hardware Monitoring
I'm no expert on this but I know this for sure since I've messed with fan speeds for both of my GPUs before with temps lowering as I upped either fan speed. But the thing is that you're also using Windows Vista so it could be different. As for which is the correct reading...focus on ATITool and Rivatuner for accurate temps. Both those and Ntune gave me the same/very similar temps...while GPU-Z was was off .
You probably shouldn't have to worry too much on temps exceeding 60C since I read people idle around those temps also. However, if you start having problems that may have to do with heat like artifacts/restarts/BSOD/freezes when overclocking then you may have to find some better cooling/airflow. The results of better cooling does effect your overclock stability. Example...stock cooler/56C Idle/Couldn't even pass 650mhz...aftermarket cooler/42C Idle/Overclocked at 687mhz now. Hope this helped you out a little. This is the first time I've given a long explanation like this lol .