LoL should be running just fine on the rig you have now. Assuming you mean Black Ops 3 as the other game, the rig could use a stronger video card for that.
My humble suggestion:
Zotac GeForce GTX 960 4GB AMP! Edition Video Card ($194.99 @ Amazon)
It needs an 8 pin power plug instead of the 6 pin on most 650 ti's, but it has almost exactly the same power consumption, so as long as your PSU has an 8-pin, it'll do just fine.
Cheers!