I can tell you are going for SLi eventually, and I can tell you that you will likely be disappointed in the results for the amount of money you spent. Having said that, the card and PSU is fine for dual SLi, but is marginal for triple, and a no no for quad. You could of course save some money there by getting a single GTX 580, and drop the PSU to 650 watts.
You can view this Tom's chart on cards which includes SLi and Crossfire ratings and judge for yourself. My personal opinion is multi-card is not worth the expense or trouble for the gain you get. In most cases you can do just as well with a single powerful card. For me the only reason to run more than one card would be for a multiple monitor setup, or workstation cards, etc. http://www.tomshardware.com/charts/2011-gaming-graphics...
OC can improve performance, but not as much as you think. You are not going to get double the stock speed. OC can also vary from board to board and component to component, so if some other person gets 20 more points in 3d Mark with the same setup, don't be surprised. It does not mean you failed in some way. His components might just have had a little tighter tolerances or something. Such is the nature of the beast known as electronics.
You can spend hours viewing articles on Tom's and other sites comparing OC to stock speeds. It's up to you. I can tell you the i5-2500k is pretty fast at stock, and even faster when turbo kicks in.
Since this is your first build, I would simply start by getting a stock, single card rig up and running. You can worry about OC, SLi and such later.