Need Advice for budget First-time Build

Hello I am looking to build my first computer and I could use some advice. I have been looking at parts for a long time and could use some external suggestions. I want to build a desktop computer capable of playing the latest games as well as upcoming games within the next couple years without much issues or lag. It needs to not have any issues running two screens as well (for tasking not gaming) A goal for this computer would be to have a desktop that I can expand on as time goes by (add more hard drives and more features) so I was thinking maybe I might need a full tower for more slots for things (correct me if I'm wrong). Maybe one day I might want to turn it into a multimedia station for movies and tv.

Problem is I only have about $700 dollars to spend (I could probably budge $100 move). I have already bought a powersupply (750W) and a copy of Windows 7. as far as other components I was looking at the i5-2500k, in case I want to experiment with some minor overclocking in the future, and I was thinking about buying a hard drive around 1TB for now but if I need to settle for less its fine.

Could you guys help me out? Is the i5-2500k too much for my budget? (or would it be smarter to buy a different cpu for what I am trying to build?) What would be a good GPU to buy with my budget? Should I try to focus on Nvida or AMD? Is going for a full tower a waste for my purposes? If you guys could help out I would be very thankful, its a lot of stuff to consider if you arnt used to considering it.