Two 750?
SLI/Crossfire is only a valid solution if there is no reasonable stronger GPU. I'm sure you've also had a few Problems with games not working as they should or drivers missing.
Btw we have 2015 now. Why a gtx750? Gtx 950 is the new generation.
And if you need two of them you're around 300$, that's enough for a AMD R9-390.
At the same price range is the nVidia GTX 970 but as you might have heard NVidia only uses 3.5gb+0.5gb on this card. This MIGHT not be a problem or MIGHT become a problem. You can read a lot about that on the Internet. Still it's not recommended to buy a partially broken product.
Oh yeah, and I don't think you will get more than 10-20% FPS plus from two 750 over two 470. The Thermi Chip in the 470 was fairly hot but also somewhat powerfully. But that's just my guess and no validated fact