The 2500K offers 95% of the 2600K's performance, but at 60% of the cost.
The GTX570 is a solid upper-mid range card, a pair of them in SLI would cost $700+, but SLI is not really needed normally unless your monitor is larger than 1920x1200. The GTX580 is the top dog in 90+% of most games, but, costs $500 or so...; as a single card it is fast enough for all games, save for perhaps Crysis at 2560x1600 at very high AA/AF settings.
Agree with mdd1963 that you don't need to SLI to start. You're actually doing a better job of future-proofing if you buy a big graphics card now and allow room for SLI down the road. In the end, most people don't end up getting a 2nd card of the same generation, usually by the time a 2nd card is needed, there's bigger/faster/newer/better out.