PCNoobXCVII :
I currently have an MSI R9 270x and I was wondering when I upgrade what should I go with. Should I get a card that's crossfire compatible with this one or should I just switch to a Nvidia card(If I do this I will probably go with the GTX 780)?
the two options are completely different, an additional 270x is cheap compared to a 780.
an additional 270x will give you a boost, perhaps between 40-80% additional fps, when xfire works (which isn't always, watchdogs being an example).
Generally single card is better, but as an upgrade path xfire is a good choice.