Quad Boot
I use Snow Leopard on my Gigabyte GA-X58A-UD7 based rig, it's faster than a stock Mac Pro Quad core and can be overclocked.
I Run a quad-boot system
1) Snow Leopard
2) Ubuntu
3) Windows 7
4) Snow Leopard Backup
Three of these are on the same hard drive (remember the old rule: partition, install windows, then install everything else). The booting is with PC-EFI which brings up a nice menu at boot to pick your start up disk.
Performance:
Snow Leopard is snappier as far as a user interface (the main reason I use it), window management is better than Windows, unix commands and linux interoperability are built in and it is less buggy and less prone to being broken by software installs.
Windows 7 performs much better with 3D games (mostly because of code optimisation).
Linux is a leaner OS than both and if you need to do real-time tasks it's the only way to go (you can compile programs into the kernel at a higher priority than allowed by OS X or Windows)
Sound processing is better on OS X due to core audio
Have a look at the insanelymac forums, you may find that your existing hardware is capable of running OS X, just buy a copy of Snow Leopard, check out the forums on how to install for your hardware and put it on a separate harddrive if you're not familiar with the black art of partitioning...
Also, don't get sidetracked by Apple fanboys who say this is illegal, it's your hardware and your right to do with it what you please, as long as you buy the software, you have committed no crime. The EULA is an agreement between you and Apple that basically states that they won't support you if you install Snow Leopard on non-approved hardware, fair enough, don't expect support from them. But their claim that you are not allowed to install this on your own hardware is marketing, not law. No EULA has been supported by a court where it inhibits your right to use software as you see fit, actually the DMCA reinforced this right.
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