It depends on the GPU, but normally... loading at 80C is acceptable.
Though we won't need to tell you that you'll need better cooling for other components.
For instance... the HD4870 is making my CPU idle at 55C (no joke), but on maximum load, the CPU does not exceed 65C, which is perfectly acceptable.
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cChip interpreter WIP - current status: Release Candidate
LRx Filter RC - current performance rating: 9/10