It's SLi for nVidia cards. To answer your question, yes, I would recommend getting a new card. The 8800s in SLi will clock in around the 560 performance-wise, which is nVidia's mid-range card. Plus of course it will move you to a DX11 environment. The other problem is that some games have issues utilizing SLi fully, so in the worst case older titles may not see the card or even utilize a good portion of it - in that case it would be like you had a single card OC'd a bit.
When people ask about SLi/Crossfire I always say "make sure you have a powerful enough card that if the game didn't recognize the 2nd card at all, you'd be fine on the single", and I believe that's a very safe route.
Also, when you go SLi you need to make sure your motherboard supports it (two PCI-e slots), which if you have a PC built by a manufacturer is incredibly unlikely. Equally, you need to make sure your power supply supports it. While most aftermarket PSUs will support a single dedicated card, SLi is a different story - you need to make sure you have 4 6-pin PCI-e connections, that the wattage is high enough (and consistent enough), and in particular (what many people forget about that) that the ampage on the 12v rail is good enough to handle both.
Last edited by Vilgent; 02-14-2012 at 03:25 PM.