GA-EX58-UD4 help (Graphic card and ram)


I have a GA-EX58-UD4 motherboard and looking to upgrade my Graphic card and ram to meet the required requirements to run UE4.
I am no hardware techy, so bare with me

What does this mean: “2 PCI-E 2.0 x16 graphics interface with CrossFireX and SLI support for ultimate graphics performance”
If I understand this correct, I cant put a PCI-3 in right?

And what graphic card should I get? (price is a concern, so not the newest card should PCI-3 be able to fit)
I am running two screens, and it would be nice if a could run both and also have HMDI output.

I currently have a 3 x 1.5V DDR3 2GB ram block setup, so 3 slots are free (dont know the MHz).

Does the Ram Block have to be the same as the ones I have now?

Is MHz important?

The motherboard can support up to 24 GB, so what do I buy?

I hope you can help me with these questions
Kind regards