First question is yes if you are going to stay with nVidia cards. AMD cards you can use one if it supports the three monitors in other words three out put ports.
Second question is no they don't have to be the same but it is recommended by nVidia and AMD to make all the monitors the same. I don't know what cards you would be looking at and what your budget would be.
If you would like to get an idea of what cards to buy then repost your budget and we or someone will be more than happy to answer. I do wish you good luck.
The thing with three monitors is that if they are not the same size they do have to be the same resolution at least when playing a game. In destop mode the video card will support the native resoluton of each monitor. Like right now I have a 30" monitor at 2560x1600 and right beside it is a 27" at 1920x1080.
If you do plan on three monitor gaming and you are getting two new video cards then you should try to get cards with more then 1gb of video ram on them and don't connect all three to the same card. That will help with distributing the vram on the two cards.
You also want to make sure that your psu is big enough to power two video cards.
Thanks for the fast reply's! The information is most helpful... Two things that I need to ask. Most of the better quality NVIDI cards that are out now, they have the designation of FERMI but I thought there was to be an upgrade to these and that was suppose to be out by now? Finally, flipping to the other side of the fence, isn't the ATI or AMD cards suppose to handle the three monitor setup better? Would you have a recommendation for one or two of these coasting somewhere between 150-200 each with 1gb each?
For a 3 monitor setup 1GB of VRAM for each graphic card is not enough. 5760x1080 resolution means a lot of textures will need to be loaded onto the video cards. The VRAM on each card do not add up so if you have two cards with 1GB of VRAM, in SLI or Crossfire setup that means you still have only 1GB of VRAM. That's because the same textures are load onto each card.
I just have a hard time switching to ATI or AMD I have been a loyal NVIDA user for years and I'm still reading up on which is better Eyefinity or a couple of NVIDA cards?
If you intend to get just 1 x 6950 (must be 2GB Version), it will struggle with a triple monitor set-up at 5760. It will achieve 30-40fps only. You can try it & see if its good enough for you. If you want better fps, get 2 x 6950 to achieve ~60fps in most games.
Your power supply should ideally be 750Watts for 2x6950.
Lastly, better get all your monitor of the same size. It will be odd looking at different sized objects across differently sized monitors. Ideally, it should be the same brand & model to ensure they are of the same height so the pictures align perfectly when put on the same table & they should have narrow bezel to minimize bezel distractions (although such bezel distraction go away in the heat of the gameplay).