Second graphics card on motherboard - only same chipset? True, and if so why?

Question

I am planning to buy a new motherboard (Asus / Intel i7). Since I already have a decent VGA, my plan was to continue with my graphics card for a while and then buy a second (high end) card. I want to use both cards then. However, my computer dealer says, I cannot mix NVIDIA and ATI Radeon cards.

1. Is that true, and if so, why?
2. If true, is there some other restriction or is it just the vendor. So can I mix any Nvidia with any Nvidia?
3. Or is this just wrong, or might it depend on the motherboard?

I am aware that I can use advanced features if I provide two identical graphics cards. So it is recommended to use similar cards - but is it a must. BTW, I am talking about a WIN 7 x64 system, if this matters (restriction OS based?).

-- Edit --

After simkill's answer (pointing in the right direction) I have found this illustrative setup quide (link).

Answer 1

You can use both ATI and Nvidia cards in the same system if they are working independently. To work in SLI setups, you will need cards of the same model, but the SLI performance will always be equal to the lowest performing card. For example, you can run a a factory overclocked Nvidia 680 in SLI with a standard nvidia 680, but the OC 680 will downclock to the other 680. This is the same for attached memory. If one card has less memory than the other, the SLI will scale to the lowest memory amount.

It's also possible in SLI to run a third card that is not the same model, but it must be dedicated to physx. I was going to run 2 460's and a 9800gtx and dedicate the 9800 for physx, but after researching, it appeared the 920 i7 processor I have would do a better job so I left out the 9800 from the setup.

In Crossfire with ATI, its slightly different. Instead of having to run two of the same model card, any card in the series will run alongside another in crossfire setup, so you can mix and match 7xxx model cards for example.

Answer 2

You can mix graphics cards from different vendors without any problems (given that your Motherboard has the approriate PCIe slots), you cannot, however, make them work "together" as in using both cards to double your 3D performance.