You are better off getting the most powerful single-card you can buy (we cant say "single-gpu" anymore, since all new cards already have several gpu's in the same "shell")
SLI or Crossfire doesnt give you twice the preformance, so it is simply not good value for money. Add to that the fact that very very many games dont properly support it, and will show next to no preformance increase since many games only utilize one of the cards.
So dont bother upgrading your motherboard and reformatting your entire pc. Just get the best single card you can get, whatever brand you wish, and you'll be happy.
Unless you wanna play in 4k with supersampling on top of that, a single card will be just fine for 60fps maxed out gaming.