You could use any acoustic dampening foam, like the ones we use in the studio, if your PC is too noisy, really. It doesn't have to be Auralex, though...

On a more serious note, density of the foam is the most important when it comes to PC cases. You could actually use the foam they use for sealing heat pipes and electrical pipes I believe... that's the cheapest solution. It is very dense, and easy to apply because one side is sticky.

 

Yeah, I became concerned because when I brought my PC into work to get Basstard, one of the guys commented how loud my fans were.
Turned out it was the CD Rom spinning up where I was loading some drivers for a Bluetooth dongle, haha.