What resolution are you going to play at? Up to 1920x1080 a 580 will go full ultra+4xMSAA and stay 60+ fps basically all the time. At 1920x1200 and beyond you'll have occasional dips with 4x MSAA and ultra settings, down as low as 45-50.
It will run on mostly on ultra, if you have not brought yet, then a 2500K overclocked and 2x 570's will run it more fully on ultra, or 1x580 performing to the same level.
SSD's gives you faster loading times, but thats all.
To answer the question about SSD's, I have personally asked the same question and when I finally purchased one, I did notice better performance with an SSD over a 7200/64 hard disk. Running games fast? The load times are incredibly fast, notably more for singleplayer types as you must wait for server connections on multiplayer. Hope that helps out.