You need to use an additional video card to get the triple monitor setup working. Additionally, if you wanted to use SLI/Crossfire in this setup, you would have to check Nvidia's website to see if triple monitors are supported (e.g. I had 2 x 8800 GT's in SLI, and had to disable SLI to get triple monitors to work - this does work with some newer Nvidia cards however). See this page for more details.
Using two video cards in non-SLI is a completely different story, and quite possible with the 8600 GT you mentioned. My recommendation is to use your one center monitor as your "main" monitor (and have the 460 GTX be that monitor's hardware acceleration), and plug the other two monitors into the 8600 GT. That way, 3D applications that output to your main display won't take a performance hit (those running on the additional monitors will - you might want to put one of them back on the main 460 GTX if you need something else hardware accelerated).
The core requirement from Nvidia's website to use an additional video card to drive an additional monitor is:
Additional monitors (up to 6 monitors total enabled) may be enabled by using either a motherboard GPU and/or a PhysX capable graphics card (GeForce 8 series or higher with at least 256MB of memory) that does not have the same GPU as those that are SLI enabled. More information regarding multi-monitor in SLI can be found here.
The above information applies wether or not you use SLI.