If this is your setup then you should be lucky that your GPUs are only at 90°C !
They should be melting by now!!!!!
Very bad unfortunately.
Notice where the fans of the GPU are and try to think how cool air from the outside of the case, will manage to reach that area. It will not because the two big fans down and up will channel the air flow towards them NOT towards the centre of the case where the GPU fans are.
You need to put a fan on the side panel, just there where the GPU fans are, pushing air into them. If I manage, I will find a picture of my old rig to show you what I mean.
Edit... got one picture, it is a bit old (from 2006, it is two 7900GTX on the picture) but it shows what I mean: A big fan (slow rotating) throwing air directly into the fans of the GPU (the reason for this setup was to throw air to the chipset fan which can hardly be seen between the two GPUs which was not effective enough and had to be watercooled, but that is another story).
It is my opinion (and you do not need to pay respect to it), this case is trouble...
Anyway, you can only check with a thermometer in the area of the GPUs all the rest is pure speculation. Get one of those house thermometers with a small sensor for the outside the house temperature. You put the small sensor near your GPUs and you can easily measure the temparature from the display outside.
Lot's of fun!