I wouldn't unless you have a 30" 2560x1600 resolution. About a year ago i purchased t 570's and sli'd them for BF3 and i wasn't happy with how they worked. I eventually pulled one out and used it on another system. I didn't like it and should have saved my money. But thats my experience
What resolution are you talking about? If it's just 1080p, you'll be fine with just one. If it's a 1440p monitor (2560x1440) then SLI/CF might be an option although multi-GPU configs introduce a lot of potential headaches so I would recommend getting the monitor first and trying only one graphics card first. If you are still stuck for performance you can always buy a second graphics card at a later stage.