I am putting together a system that will (hopefully) do both games and CAD (SolidWorks). I have found many charts on gaming cards' performance in workstation apps (and they fail miserably compared to workstation cards). But, I can't find a single chart for Workstation cards' performance in games.
The issue with cards like the Quadro is that the drivers were not written for speed but for accuracy. So while the core chips may be the same for the Quadro version of a card and for the consumer GeForce one, the perfomace is just not there in games.
I'd say you will loose 25% or more in gaming performance going with a comparable pro card vs. a gaming card. I have a laptop with a Quadro NVS, and it does not do too badly in gaming, but falls behind my other laptop that has a similar generation ATI consumer card. I can run WoW on the Quadro at about medium settings with 30-40 fps in most areas. My son also plays Battle for Middle Earth II on it, woks pretty well, no lag on it that I can see.
It's probably not very good. As far as architecture I'm sure there's some differences, but the main thing is drivers. Drivers are made specifically for their job. Nvidia, for example, makes gaming graphics drivers that are optimized for specific rendering engines and both companies optimize for specific games. The workstation card drivers completely ignore gaming requirements and focus on profesional apps like CAD, 3D renderings, animation etc.
It would be interesting if you could switch drivers tho, and see if it works...
There are some hacks around to enable gaming cards to use pro drivers, which should allow switching between the 2. However from a practicality and stability point of view, that's probably not a good idea to do. Just saying that something like this has been done.
Crossfire, no, but you could run both independantly - in which case you might consider that FirePro with something like a 5770 (to save cash). I think the biggest hassle of that is that you'd need to switch the monitor connection or something, to launch games off the gaming card and CAD off the workstation card. Not entirely sure how much fan that would be...
Otherwise if you only run SolidWorks and if you generally don't use huge assemblies, you might get away with a 5850... I haven't looked at benchmarks but if the 5850 performs decently then it might be worthwhile should you want to do some gaming too.
Workstation cards would perform pathetically in games but mainstream (gaming) cards would not perform that badly with workstation apps.. Thus, you are better of buying a mainstream card.. As with your build, get an i7 860.. Apps like Solidworks do benefit from hyper threading.. For the card, get the GTX 460 1GB or HD 5850 1GB (both would perform identically so it comes down to your choice on ATI vs Nvidia)..