Regarding Nvidia cards:
2. I also read somewhere that one of the major differences between Gaming and Workstation cards was the Clock Speed - faster for games, slower for greater stability for long/complex renders (and another point of difference being the level of driver and hardware support for workstation cards). My card is an EVGA-branded model, and came with an app that lets me adjust Clock Speed. I haven't used LuxRender that much, but in the two renders I tested it with, when the card was running at "default/gaming" speed, LuxRender crashed after some time, yet when I backed the speed down to -40, it was more stable. Has anyone else tried this with their cards? Do other "brandings" have such an app (PNY, etc).
3. Since my card is EVGA branded, should I get a 2nd EVGA GTX760 or can I use, say, a PNY GTX760? What about the VRAM? Since mine is the 4GB model, will a 2GB model slow things down or give me 6GB total (in SLI)?
Workstation cards (like Quadro, Tesla) are for 24/7 usage.
They have a way longer lifetime when used 24/7 for rendering compared to consumer cards (Geforce).
Their architecture to deflect hot air is different and better. They can also handle way more OpenGL data. So your viewport while modelling runs way fast if the scene is quite full.
These are things a consumer or gaming card doesn't need that much. For gaming you don't need that much OpenGL data, and they usually don't run at full load for a longer time.
Got a test with between a between an older quadro 4000 and a gtx780 with 3dequilizer (camera tracking software).
The 780 was still noticeable slower calculating the scene than the old quadro.
So it really depends on what you are doing or what you want to do. The prices for the quadro series is high, but if you are going to build up a 24/7 gpu renderfarm, it is the better choice. But even hollywood stays with cpu renderfarms because it's way easier (and cheaper!) to build multi-core server cpu workstations, where one machine has 28 cores or something.
As long as it is another Nvidia card which uses the same driver it should be no problem.
But is is even possible to have an AMD and Nvidia card in one setup, but for this you might ask Gazukull, he has done something like this
Having a Quadro and a Geforce card is also possible, but you need some workarounds for it.