Identical
I believe there's no difference as far as output goes. You definitely won't notice a difference. The benefits of the 1G card is, of course, being able to add another 1G card to the empty slot. And trust me, you're going to want to do that eventually... the greed/hunger always sets in eventually.

But the benefit of the 2 512MB cards is price. Much cheaper than a 1G card. Which is why I went this route. I could have a 512 delivered for $100. Whereas the originally price for the 1G that I wanted was $450! (now down to about $360)
-Ransom