Okay guys, first, I am very new to all of this. I actually built my computer for photo/video editing.  I am running two of these cards to help speed up while editing video/photos. I need to run three monitors. I have one monitor vertical between the two horizontal monitors.  I like this configuration.  Adobe, and Pinnacle are both very taxing on the system.  From what I can tell, from all the research I have done. I would have to pickup a third GTX 460, put two in SLI and one on its own. to keep mu current 3 monitor configuration. Current build is below. Nothing has been over clocked.
 So my question is what would I gain by going to SLI? Is SLI the only way to get the most out of my system? is the current configuration going to give me what I want? Are these even the correct cards for my use?
 I have searched high and low, Any help is much appreciated. If I am wasting money please let me know.

Vista Home premium 64 bit
Asus P6T  LGA mother Board - intel X58 chip
Intel i7 920 2.66 Ghz
12 GB  ram - Corsair/1066
2 EVGA GeForce GTX 460 video cards, Not in SLI (EVGA 01G-P3-1373-AR GeForce GTX 460 (Fermi) Superclocked EE 1GB 256-bit GDDR5 PCI Express 2.0 x16)
Corsair liquid cooler - smaller of the two.
Cooler Master Silent Pro 600w power Suppply.
 I have a Corsair 750w on order
 

derekdub

You should be fine with just 2 460's in SLI. SLI would be a huge boost in multi-monitor support. PUT THOSE 2 CARDS IN SLI! Then configure the triple monitors and you should be fine. If you do get a third card.... get a triple SLI Bridge and do that. In my opinion though, the 460 is a powerful card and you should be fine with just 2 cards in SLI.

You can not put a Tri SLI Bridge on the 460. It's Impossible as the card only has One set of SLI Tabs on the PCB.
 
@OP You will be fine with two cards. You will have two monitors hooked up to one card and the third monitor hooked up to the bottom card. The top card may run a little hotter as the drivers require it to run two monitors. Unless they changed something I am unaware of.