As "dipankar" says its not a problem, normally.
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.