SLI or crossfire is not worth it for low to mid end GPU's. For example when you want more graphical power than the high end-ish graphics cards like the R9 390X or GTX 980 then you could consider an SLI GTX 970 or Crossfire R9 290X or simply dual the most powerful graphics cards.
Because of the lower end specs it is not advisable to grab two GTX 750 Ti's for example, because the specs don't double up. The first graphics card does one frame and the second one does the other.
For 330 dollars, either the GTX 970 or the R9 390 is your best option.