I think its a little too hot, if you can't keep your finger in the back of the GPU for more than that time the temp must be around or over 60C, and when 3d gaming tems usually climb onether 5 - 8C more. its not a big thing as long as it doesn't give you problems
If the GPU heatsink is warm you could just install a new fan and that will help to cool things down a little, for better temps and peace of mind it might be worth it.