Good point/question.... one of the (many) things I love about my MP is that it is silent compared to any of the gaming rigs that I used to build and upgrade. That said, I think cases and quiet cooling have probably come alone some since the last custom PC I built for myself back in '07.
I recently bought an i7 system from a fellow member (for a friend) and he says it is VERY quiet, but again relative to his circa 2004 gaming rig.
If it were possible to build an i7 system with full functionality as a Hackintosh I'd probably consider it and even did with the system I picked up here on MR, but not sure if the latest X58 chipset is fully supported, etc etc. The MP is definitely not cheap, but it's also a different class of hardware... server grade xeon procs/chipset etc. This has been discussed to death here, but comparing a Mac Pro to a gaming machine is not an apples to apples comparison (no pun intended). That is unless one built up their gaming rig with workstation class dual socket xeon's, etc... And then the price gap is pretty close. The higher end Dell Precision workstations compare well with the MP architecturally and those can easily exceed the cost of a Mac Pro....