Given you use this system to earn a living I would steer clear of building a hackintosh. Simply put installing OS X on a non Apple computer is prohibited.Sorry, I wasn't clear enough with what I wrote and omitted the word Hackintosh in reference to earlier comments. On paper they deliver much better performance for the same money, however there are clearly some caveats both legal and practical to purchasing one.
I hope you didn't read my comment as to suggest you would do so. I was referring to some sellers I saw. For example:I have never actually sold a Mac, since they are invariably passed on to family members and used until the die, but if I was selling one I would never dream of including any software beyond the basic OS. Nowadays, I even wonder about that, since it's tied to your Apple ID.
http://www.ebay.com/itm/EIGHT-CORE-...629487?hash=item33abdba06f:g:lCAAAOSwMmBVkCzd
Clearly the seller is attempting to use the installed software as a selling point for the system.
As for OS X that's not a problem for the majority of situations. The licensing terms allow anyone to install the latest versions of OS X on any Macintosh system for which the OS is supported at no cost. It's not tied to your Apple ID (though that's how most people obtain the new OS). If you were to hand your current Mac down to someone else feel free to install the latest supported OS on it...it's perfectly legal to do so.
IMO a small price to pay in order to ensure I can continue business should my primary system fail. Given you don't appear to have a reliable / convenient means to service your existing systems the low cost of a backup system, even if sitting on the shelf, is well worth it. How much would it cost you to repair your existing system (depends on the issue but could easily be more than the cost of a backup system)? How much time would it take to get it repaired? What business impact would it have while you are working to have it repaired? What would be the impact to your businesses reputation as a result? IMO $800 is well worth the peace of mind having a backup on the shelf would bring. But this is only something you can decide...it's all about risk management.Looking at backup options it's now clear that I would have to spend a great deal of money to buy something new that beats this Mac Pro on heavy multi-core tasks. From passed experience, backup machines tend to remain unused unless the main one dies.
My suggestion would be to buy a used, backup Mac Pro to have as a stand by. Given how inexpensive they are you can buy an equivalent system for a very reasonable cost. If that's still too high then perhaps you don't need as capable of a system (it's not uncommon to have a lesser configured system for use as a backup) which could drive the cost even lower. Plus you can use the backup system to troubleshoot the primary system. For example if you had a lower end backup Mac Pro you could grab the video card out of it and use it to test your primary system. Right now you're speculating the video card is the problem but cannot be certain. If you had a backup video card you'd know whether or not that is the problem. Plus you could continue to use the primary system until the replacement card arrives.If I were to buy a new machine, the 15" MacBook Pro, though massively expensive, would offer decent performance and I would be likely to use it, so it wouldn't just sit there gathering dust and a lot of the data could be synched. The other option is to by a cheap secondhand Mac Mini that can be used as a temporary measure when the Mac Pro is no longer viable.
Finally there's the peace of mind of not having to deal with a failed piece of equipment at an inconvenient time. You could perform the troubleshooting / repair at your convenience and not a mad rush as you have to keep your business going.
Good luck!