It depends on the games you play.
If you ARE limited by the VRAM then a couple GTX580's would solve the problem and give you slightly better performance.
If you're NOT limited by the VRAM then moving to a couple GTX580's will be of no benefit at all.
There's an option on EVGA precision and MSI afterburner to monitor VRAM usage on the GTX500 series.
Best thing to do is play a couple of games with the VRAM monitor enabled and see if you really are running into VRAM limitations