My specs is quite similar to yours, and my XFX 8800GT Extreme 512MB @640/1900 is also idle at 58-59C. When full load under Crysis, it will reach 70C in my Antec 900 case. It's normal and I've asked XFX help desk and they said it is safe until 120C then the damage will incur.
If you intend to replace it with aftermarket cooler, you can try Thermaltake Duorb, heard it the best 8800GT cooler until this very moment.
58 thats normal.. i hang around 55-58 on the norm.. and when gaming i will see 71. your card can withstand alot more, i know they are stable up to 90 C and the evga can do 100 C. if not mistaken
Really only 70C when gaming? What make, and do you have your fan settings above the default? Also by gaming, what game/settings/how long? Mine hit somewhere in the 80's with everything stock and some Hellgate London after an hour or so.
If the BFG 8800GT OC 512MB reaches 80C while gaming for an hour or so (at stock speed), you will need to relieve the heat from within your PC case.
I have the following:
1 PC fan in front to draw in fresh air.
1 PC fan on the side panel to draw the heat out that the video card gives out.
1 PC fan in the rear to draw the heat out and assist with keeping the flow of fresh air coming in.
1 CPU fan
1 large PSU fan below, 1 80mm PSU fan on top to draw in fresh air.
Hope this helps...
FYI, the max under heavy load mine will reach is 71C....used to get close to 80C before the fan configurations.
My idle temp is 41C with the fan at 30%, increasing to 60% drops it down to 37C - 38C- Rivatuner rocks!