You need to replace the fans. The original fans are
extremely weak, and are probably the worst 120mm fans I've ever seen. But yeah, you're going to need some serious CFM if you want the traditional front-to-back setup to work.
The three front 120mm's need to be high static fans since it's being met with the resistance of the front grill, dust filter, and of course your graphics cards. Those two 80mm puppies in the back have to be high CFM to equally exhaust the front fan's intake (minus PSU exhaust). Any less, and it starts becoming a hotbox.
I have NF-F12s for my 120mm, and NF-A8s for my 80mm. If you're feeling particularly adventurous, use
these for the 80mm fans.
I'll let you know how well my setup works when I get my bracket...