I am afraid you are not going to be doing much gaming with a GT 520 card. It is very low end and really designed more for an HTPC and as a replacement for integrated video that has pooped out. If it is not too late, I would return or exchange it. You can look at this Tom's chart. For gaming these days, the Nvidia GT 545 or a AMD 6670 would be the absolute low end. just get the most the budget will allow. http://www.tomshardware.com/reviews/gaming-graphics-car...
Since budget is a concern, I would also skip the i5-2500k processor. You can get the Core i3-2100 for now which is still quite fast, and upgrade to the 2500k later. This will free up money for a much better video card and a good power supply to run it.
Since video editing is also in the mix, I am going to suggest an inexpensive ASUS or Gigabyte z-68 based motherboard. This type of board has features that will aid in such things.
Also for the video editing, I would suggest 8gb ram(2x4gb kit) DDR3 1333 or 1600 speed and 64 bit Windows 7.
For the video card an AMD 6850 would be very nice. Not the top end, but will do well in most games without breaking the bank.
And finally a power supply. A good quality 550 watt unit from Antec, Corsair, Seasonic, Silverstone, and XFX with at least an 80 plus certified rating(Bronze is even better) would be best. If it is not certified avoid it.