Render nodes

Hi all,

I am looking to expand my render farm (by between 2 & 4 machines depending on cost) & would appreciate some advice.

I currently have 8 Mac Pros ranging from the first quad 2.66 up to the 2008 Octo. All Mac Pros have a minimum of 16GB of RAM, most have 24GB & a couple have 32GB. They are all running Windows x64 on a 2nd hard drive & are used for high-poly Vray rendering with 3ds max.

I originally went for the Mac Pro as it was cheaper than a comparable Dell with the added bonus of being dual-boot capable. Setting up as a x64 workstation takes less than an hour & they are very reliable even running at full power for 24hrs a day.

So, the time has come for expansion & am a bit lost as to if the current Mac Pro (or the imminent update) is still competitive against the PC boys.

My requirements are:

CPU: 8 Core (as fast as possible)
RAM: 24GB (absolute bare minimum - 32GB is better)
OS: 64-Bit Windows (x64 or Win7)
Video card: irrelevant
HD: 1 with approx 320GB is fine
***Ability to run flat out 24hrs a day for longer periods without sounding like a jet taking off (Dell Precision 650 I'm talking to you!)***

It's pointless to ask this on a PC board as the fanboys will just dismiss the Mac anyway & there seem to be quite a few people on here who know their stuff on both sides of the fence!

Any advice gratefully recieved!