The foam is WAY easier to install though, and i'd put it there even after the case-mod, if indeed it will help.
The lighter the weight of the foam the more useless it is. Go to your local home improvement store and look for things like HEAVY FOAM backed carpet tile.
To ensure adequate air flow with the least noise some measure of science is prudent.
Generally you should:
-1- test your system and record the results
-2- make a change
-3- retest and compare the results
These are good tools:
CPUBurn - processor stress software x4 for your quad core CPU
FurMark - to stress the GPU
GPU-Z - to monitor GPU temperature and fan speed
ASUS PC Probe - to monitor CPU temperature and fan speeds