cheapest i5 you can find, g2 at least. A 650ti boost or a GTX660, 8GB ram, corsair cx 430 and a b75 motherboard asus gigabyte asus or msi.
4 gigs of ram will do if you are feeling cheap as will a 650ti, even a 650 is enough but if you want it to work well without strain or gpu bottle neck get the 650ti.