I just posted this on a similar thread. You can over clock without any voltage bump for quite some time. My GTX 580 is a EVGA SC version. BTW welcome to OCN !
Acceptable is settling for less than perfect. If it's got errors, you'll either need to back down clocks or bump up voltage. Don't pass 1.1v on air however. When you reach top clocks without errors and stable system is your GPU OC limit. All GPU's OC different limits and luck of the draw.
My default voltage is 1.0872v. I found out that default voltage can vary from GPU to GPU. I was always wondering why other people would get lower temps than me at stock core default voltage. I currently lowered mine to 1.0750v which brought down my temps by 1C and still run stable.
Here are some of my 580 SC clocks to help you.
The GTX 580 SC is 3% gain higher than a GTX 580 vanilla at stock speeds.
39-40C idle @ stock
797Mhz core on SC which is a 3% gain over vanilla stock.
40-41C idle @ 838Mhz core on SC which is 8% gain over vanilla stock. 838 Core
41-42C idle @ 855Mhz core on SC which is 10% gain over vanilla stock. 855/1086/1710
42-43C idle @ 875Mhz core on SC which is 13% gain over vanilla stock. 875/1111/1750
43-44C idle @ 890Mhz core on SC which is 15% gain over vanilla stock. 890/1135/1780
To go higher stable I have to bump my GPU to 1.1000 voltage.
44-46C idle @912Mhz core on SC which is 17% gain over vanilla stock.
Now when I OC I bump up across the board for Core / Memory / Shader to be exactly the same percentage gain on each GPU-Z validation.
I got clocked to on my final score 3D Mark 11 mid level performance score of 6902. On an i7 950 @ 3.8Ghz OC - GPU 1.1000 voltage @ 912Mhz core.
3D Mark 11 score of 6902
Temps maxed regardless of over clock when I play Crysis 2 max as high as 82C and with the 580 they can handle up to 95C before I'd start to worry easily, though they shouldn't reach that high on single card set ups.
PS. Those are the actual clock speeds. Open up GPU-Z
. MSI Afterburner has settings clocks differently please note. In other words when I say 875/1111/1751 for GPU-Z which is acurate clocs on Afterburner those clocks show differently at 876 Core / 2222 Memory / 1751 Shader.
In case you didn't know the term 'vanilla' is EVGA's stock card. 'SC' is super clocked which come factory over clocked higher than the vanilla's. I originally thought it was better binned VRM if they came over clocked already since they garrantee the warranty. It may have been false beccause Vanilla cards over clock just as well as SC cards. So for you those clocks might mean different percentages of over clocks. But your card is similar in 580, I just don't know much about the Zotac brand not being as main stream.