The 4gb 270x is pointless. The gpu is not powerfull enough to utilize all that vram by itself. You'd need to be running crossfire and at that point you'd still be better off with a single 4gb 290x. For how much a 4gb 270x costs you can usually get a faster 280x. You will also need a psu upgrade. For a 270x you'll want a good quality 500w psu, for a 280x 550w.