You should be able to overclock the E8500 to about 3.5GHz, but not much farther past that unfortunately. Older Intel CPU's were never great at overclocking (and AMD chips still overclock better).
If you want to buy a graphics card that will run Crysis 3 ultra @ 1080p, you should get a GTX 770/780 or an R9 280x or R9 290. However, any of those cards will be severely bottlenecked by the E8500, so you could use it, it just won't be fast. When you upgrade to the i7 you won't have any bottleneck.
What power supply do you have? You should have atleast a 600w PSU (preferably XFX, Seasonic, or Corsair) if you're going to use a 770/780/280x/290 and add an i7 later.
Also, you might be better off buying a new motherboard and getting the i7 now, and using the i7's integrated graphics until you get a new GPU. The Intel HD 4600 graphics on the i7 are very good! The motherboard that you have now is only compatible with DDR2 RAM, which is gonna bottleneck the i7 in the future. I would recommend getting a Z87/97 motherboard and atleast 8GB of DDR3 RAM.
The E8500 is pretty good at overclocking, on air you could maybe get it to 4.1GHz.(I have one xD)