Please help me with answers for those questions:
- on a video card which specifications' say <Max resolution: 2560x1600 pixels> does it mean I can't connect 2 x 1920px screens? If I want to do that, does the card need to have max resolution of 4096x2160px?
- if I put a video card with PCI Express x16 3.0 on a motherboard with PCI Express x16 2.0 will it be a huge performance drop?
It means that it cannot support higher than that resolution via one port. You can connect as many displays of the max specified resolution as the card allows (usually 2 or 3, never heard of card that doesn't support multiple monitors).
There will be no performance drops on 2.0 motherboard for 3.0 card.