Anyways, the card itself will let you know if it cant handle the temperature over time (shuts down) - but when that time comes, it might be too late (in worst case).
Personally I prefer to keep the temp below 75, if possible.
Whats the temp on the other card? If I understand you correctly, the "hot one" gets warm due to its location and not because of workload. If so, it should be pretty easy to fix this with better air flow inside the case.
Im not a huge fan-expert, but Im sure others have good advices regarding case fans if you worry about the temp.
Yeah, I have a Haf 922 in an open area with the optional 200mm fan installed.. The ambient temps from my previous case were dropped by about 5C. The other card Runs at 65-70 while the 'Hot' Card runs at 70-80 (rarely hits 80, but maybe after like 4 hours of gaming it will).