some good advice so far, here's a few more things that worked for me...
- I pointed my air conditioner towards the tower, it's about 5 feet away but it seems to help a lot.
- also I have the side fan blowing in against the cards (I haven't tried the exhast method yet, but it definitely sounds like it's worth a shot) and my case (antec 1200) has a tunnel thingy to help aim the air from the front right onto the cards as well.
--- biggest difference I found was removing the dust filters from those 2 intake positions (other 2 intake fans still have the filters) yeah I might need to clean out the dust more often but that one change reduced the heat on the hottest card around 15 degrees
- give those plastic heatsink shrouds a closer look. Not sure what kind of cards you got, but I have a pair of HIS 6950 and the plastic shroud is almost purely cosmetic and by removing it I was able to gain a few precious millimeters of space for the intake (no spacers needed)
- also check if swapping positions might help, when I first started my top card (we'll call this card 1) was over 80 degrees C and the middle (we'll call this card 2) was less than 70. card 3 (bottom) is actually a 5850 and it will be staying at the bottom as it seems to be my strongest clocker and hashrates (Asus directCU TOP)
Once I swapped the 6950s, card 1 (now in the middle spot) is still hotter than card 2 (now at top position) but card 1 is only hitting 75 at the moment and card 2 is at 71, which I'm comfortable with - for the moment, when I get the urge to push these cards harder I will definitely be re-doing the thermal paste on both.