if you're also doing some rendering, VM's, photo/video editing, etc. go on with the 3770K. if this is solely a gaming PC, a 3570K is the most you'll ever need. it also does the former tasks just as well.
if you're in need of the whole PC components, HDD, case and PSU included, you may need to extend your budget a bit, if you're looking for a 3570K + 670 build, with the option to SLI in the future.http://pcpartpicker.com/p/GfKT
if you got a Microcenter nearby, you can grab a 3570K for ~$30 less. there's also a $77 Asrock Z75 board that allows for OCing but not much in terms of SLI. any cheaper, and it's either a non-OCable build (you lose to SLI the GPU @ 8x/8x) or an AMD build.
if that $900 budget only covers the motherboard, CPU, GPU and RAM, then that's a lot more manageable. you can freely ignore my advice on the i5 if you're looking to spend in this case.