Thank you! 
It can be different cards. For running a Nvidia and an AMD card you need a dummy plug for the unused card as the OS won’t load the driver for a unused card.
For running Nvidia cards in SLI or AMD cards using Crossfire, they have to be identical cards though, but for use with Cycles they can be different. Also, only the memory of the main card can be used in rendering though all CUDA cores on all cards can be used.
And what is beneficial with running dual cards under Windows is Blender gets a bit saggy in the GUI using Cycles for viewport rendering, though that isn’t that noticeable with newer cards like GTX560-580 than older cards like the 2xx-series even though the 2xx-series cards is supposed to be as fast (or even faster?) in OpenGL performance…
Go for the card/cards with the most VRAM, just a reminder. I’d rather have a single GTX580 with 3Gb than a dual setup with 2Gb cards…
I had to buy a card now when Adobe CS6 came as my GTX285 was just to slow in OpenCL, though I bought a GTX570 with 1280Mb’s of RAM, mainly as I got it VERY cheap but also as I don’t know if I’ll switch to Blender as main 3D app this fall or not - but if I switch, I’ll buy a 3Gb card the same day. As simple as.
Good to know!
This is my config. It’s a beast.
System:I7-2600k+overclocking fan, Corsair 500t, Asus P8vZ68-v motherboard ProGen3/16gig DDR5 Dram/64gig SSD/1Gig Sata III HD
Video: EVGA 560ti 2gig
Operating System: Ubuntu 11.04
When I get bored I can throw in some 570’s, add a bigger HD and turn on the overclocking some but right now it works and compare to production companies in speed.
Vivienne