The 3570K isn't locked. I don't think any Intel CPU is locked anymore. I'm running at 4.3 GHz on air right now and that was with an automated overclocking process. If you start messing with the voltages and other things manually, you could likely get it up to 4.8 GHz or more with watercooling.
And when buying memory, you should always go for 2 sticks, not 4. You can't use dual channel with 4 and going for 2 leaves space for more should you need them in the future.
As for SLI, I can't really recommend it. There are a lot of games that either don't support it or have issues when it is enabled (image quality and performance issues). Nvidia is pretty good about fixing these issues in a relatively timely manner but it's still annoying.