I've got 2x GTX 470s, SLi is kind of s*** if y ou have more than 2 monitors. You keep swapping between running 3 and running 2.. I assume this is solved in the 6 series because it seems to be a monitor-per-card limitation.
Mind you if you don't mind swapping between SLi and Span and go check my old thread on how to setup span mode to work like 3 independent monitors in windows (so a fullscreen window only takes up 1 etc) then you can actually quite enjoy them. TBH half the problem is my TV is connected as a 4th display so it's even more d***ing about :P
Games are awesome though using 3 monitors. all FPS and racing games run brilliantly, even 3rd persons like Assassins creed and batman are fun, but the effect is lost a little and you feel a little 'bubbly' around the edges.
Use your 60gig for windows, 120/240gig for steam+etc. Loading times in modern games is a little annoying with big beefy cards which have to load giant textures. Well, it's not, but hey you might as well give steam more space than windows. My steam folder is so massive i don't think it'd fit on an SSD.
I would go 850w if i were you as a minimum. SSD's might not require much more power, but consider later if you add another media drive then 750w is cutting it kind of thin. You don't want to reach the max of a powersupply, a good rule of thumb is to have 20% more than the peak expected wattage draw. You can probably find a calculator for how much yours expects to draw.
Again, span mode is awesome. SLi can be tempremental a little, but some of that is unique to me needing to turn it on and off to allow 4 monitors vs 3 in span, vs 1 in 1080p on the TV.
Also just FYI when you start Spanning, you really eat up your VRAM quickly. You have more pixels than people with a 2550x1600 resolution. Usually because you're at something like 5760*1080 which will probably be eating more VRAM than you technically have. So you might find yourself a little disappointed with the results considering that the 570's only have like 1280mb.
Also VRAM isn't additive. Naturally each card has to have a duplicate of the data in it's ram as it's parallel processing it. 1280 doesn't become 2560mb.