Multiple Graphics cards – need to be the same

The first place to check is Nvidia's System Requirements for 3D Vision. Without using SLI (which would require two or more of the same cards), you can't do what you're asking. This is just due to the way it works - rendering such a large image, and then sending it to another discrete video card before displaying it would be such a performance hit it wouldn't be worth it.

While this uses the 3D Vision technology, you don't have to run games in 3D mode - you can run them in 2D mode as well. In this mode, you also don't have three desktops, but rather one triple-wide desktop (at the desktop level). See this thread for more details.

Do note that the game must also support the 3D Vision Surround Technology. Note that using three monitors on the desktop level is a completely different story (if you want discrete desktops as opposed to a triple-wide one), and you can see details for this below.

Additional information: As you already know, 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 any combination of cards. My recommendation is to use your one center monitor as your "main" monitor (and have the 560 be that monitor's hardware acceleration), and plug the other two monitors into the 260. 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 560 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.

Long story short: With the hardware you have, it's impossible. If you got another GTX 560, then it would be possible.

From the manual of the GTX 550 Ti: (page 15)

SLI is a revolutionary technology developed by NVIDIA that allows you to connect identical GeForce graphics cards together to deliver increased performance.

From their SLI FAQ:

1)

Can I mix and match graphics cards that have different GPUs?
No

2)

Can I mix and match graphics cards from different manufacturers?
Using 180 or later graphics drivers, NVIDIA graphics cards from different manufacturers can be used together in an SLI configuration. For example, a GeForce XXXGT from manufacturer ABC can be matched with a GeForce XXXGT from manufacturer XYZ.

3)

Can I mix and match graphics cards with different sizes of memory?
No. For example, an XXXGT 512MB cannot be paired with a XXXGT 1GB in an SLI configuration.

So basically, you would need:

1. A similar card with the same GPU and same amount of memory.
2. A spare PCIe slot on your motherboard.
3. A motherboard which supports SLI (usually branded SLI certified).
4. A SLI-connector
5. And enough power to feed both cards (as in amperage on the +12v rails)
6. Enough cables to feed power to the card (as 6 pin connectors)
7. No more than two monitors, both connected to the primary card.

sometimes the final performance could be even worse that with one graphic card.

True. Not all games handle SLI or Crossfire well. This is why I recommend a single fast card over two older cards in SLI. However if you already have the older card or if you want to maximum speed by combining two modern cards then you can use SLI (with Nvidia cards) or Crossfire (AMDs version of SLI).