want help with pci-2.0 / 3.0

So ive been thinking about perhaps buying a card with crossfire to go with my Radeon HD 7870 but im kinda new ot the whole crossfire stuff.

So my question is do only specific crossfire card work together for example can you have 2 of the same cards or any crossfire cards will work? Also i read somewhere that HD Radeon 7870 cant crossfire together is that true?

Does anybody know a good GPU that is cheapish (100 - 200 pounds) card that has crossfire?

Sorry for the mass of questions