You don't necessarily need two separate GPU's. You can use consumer GPU's for rendering, and sometimes there is not alot of difference in speed. The main difference between consumer and professional GPU's is that the drivers for professional GPU's tend to be alot more stable, and do not change very often. Also some 3d/cad package makers do not provide support when using a consumer GPU. Although there is usually a fair amount of help available on the forums for various packages.
So first of all, research the packages to find out whether you can get away with using a consumer GPU only. If so, then get one. If you want to have both consumer and professional GPU, then get two computers. It will be far easier in the long run.
Professional GPU's and their drivers are not normally optimized for games, and you will usually get slower frame rates from them, sometimes unplayable, even though the same GPU may have been used in the development of the game.
And all that said, it doesn't really matter which motherboard you get as long as it is stable and supports the features you need. Framerates depend more on the GPU and to some extent the CPU rather than the mainboard. The mainboards can sometimes make 1-2FPS difference, but this may more depend on the chipset than anything.
PSU requirements will depend on what you put in. If yu have both a quaddro and a consumer card,like a 680GTX, you could get away with a 750 as long as you dont hav an array of 20hdds's. Mainly as the cards would not be used at the same time.
If you have SLI 680's, then you will probably want to err towards the 1000W, as there would probably be a little pressure on a 750w (it may take it if it was a gold rated PSU but best not to strain it).
For multi GPU set ups, whether SLI, Xfire or consumer/pro GPU, you will need multiple PCI express slots, usually multiple x16 sized slots. The WS board jaquith recommended has this.
and in order to switch between consumer/pro GPU's, you may be able to set applications to use a specific GPU, or more specifically, you would set the consumer GPU to be the primary (or boot) device, and the 3d/render apps to specifically use the pro card (some may just use it as a preferred device). In order to save switching cables, you want to have them both plugged into your monitor, and use the monitor's input source to switch between them. You may find you have to change the primary device in the bios for it all to work. Not too sure on that one.