I'm thinking on getting a second graphics card or replacing my current one, the R9 285. My friends have recommended the RX 480, but I don't know if these 2 cards are crossfire compatible, and if not, what card is best to go with the R9 285?
The rx 480 will not crossfire with r9 285.
Another 285(new) will be near same price as 480 4/8gb(ref. pcpartpicker).
Depending on your current psu, you may also be looking at additional cost of new one for crossfire.
As well as a number of games not optimized for crossfire.