and if i was to put 2 cards in there do they have to be the same ?
Mason: It actually adds more computing power to your computer. That's especially true if you have two of the same cards and you run them in SLI / Crossfire mode. That also answers your other question in that the cards do NOT have to be the same. It just tends to be better if they are because usually the GPU that's not doing work will just do nothing until you make it do something.
I have a desktop that has an AMD RX 480 and an Nvidia GTX 560 in the same machine and it goes just fine. They both run and there are no issues. On Windows, they drive different monitors, but they still function as one desktop. Of course, I don't get the full power out of using them both at the same time, as they're not the same card, but having them both is useful to me in another way.
I can't tell you all the benefits of having more than one GPU, but in my case, I run Linux and run a Windows virtual machine while passing through one of the cards to it. That way, I'm running Linux at full speed AND Windows at pretty much full speed, both at the same time. Windows is for gaming and Linux is for work or browsing.
TL;DR: It adds more power to your computer. No, they don't have to be the same, but you can only get their full performance benefits if they are. You can also do some potentially crazy things with more than one.
This depends on what you're going for. But generally you can purchase 2 lower cost GPUs to make a more powerfull graphics card. The issue is it's not actually 2x the power. Here is a video about it.
I have run 3 different video systems in a computer with no problem–a video card, the motherboard video chip, and a USB external video adapter.
Ran 4 screens with no problem.
The others are correct–the pair of identical chipped cards (not necessarily same brand, but same specs) can help each other. Nvidia chips call that SLI, while AMD chips call it Crossfire. They can service one or more monitors displaying one image.
Different cards can run different monitors showing different images even at different resolutions and speeds.
two cards identical can be liked in SLI [or whatever the other brand calls it], which allows it to drive a single overachieving monitor very well indeed [4k? no problem. 1ms refresh? no problem]
many are capable of operating with two different cards and separate monitors hung on each. this allows you to run your game on the better one and your out of game resources [internet supplied map? ts? etc.] in the other — where it is immediately available for reference.
or, you can multibox with far better results than two accounts on the same graphics card — up to the limits your brain can handle [in theory]
if you don't know and can't figure it out for yourself then you're obviously aren't doing anything that would need more than 1 card