You should keep your current card and buy another 660ti. If you don't have the right motherboard pay some extra and get an ASUS saber tooth, that way you can even 3-way SLI. I personally thinks it's a MUCH better option!
Hi, yeah I actually already have a sabretooth haha. Was considering the sli option, but as far as I'm aware both Mudbox and 3ds max gain no benefits from SLI unfortunately so I'm just gonna have to go with the best performance from a single card. But thanks anyway!