If you´re in GPGPU raytracing you get a truckload of GTX285 and OC them. One of them is almost as fast as a Stock GTX470 and costs 60 bucks on ebay, or get a GTX470 for 100 bucks and OC it to 750-800. You get 5 GTX470 or 8 GTX285 for what one 580 costs…
If you use it for octane, you got to wait for the beta 2.5, as the current version has troubles with multiple cards.
Also the gtx285 has only CC1.3, while from the GTX400 series on they have CC2.0 but I think Octane is downward compatible.
If not especially coded, SLI runs with alternate frame rendering under DX. For OpenGL it does nothing, unless the application is SLI enabled and has the code for it. So you´ll most likely have nothing at all from SLI+Blender.
No problem at all. If your board has more than 1 PCIEx16 specified slot you can simply combine them in Windows Vista or newer. Already did it on several occasions.
GeForce vs. Quadro:
Not going there for the 10.000th time. The GF400/500 series has crippled OpenGL support and runs extremely slow compared to a GTX285 or all AMD cards. If you´re unaware of that, use the forum search.