I currently have a single old graphics card to which I have connected 2 monitors giving me a big desktop of 2560x1024. If I get 2 of the following graphics card:
Will I be able to connect 2 monitors per graphics card, giving me a total resolution of 5120x1024? I guess what I'm asking is, will I simply be able to stick both graphics cards in, plug the monitors in and will it all just work out of the box?
I currently have 4 dvi monitors which have a native resolution of 1280x1024 each.
Quad monitors is unfortunately always a tricky proposition and will almost certainly require a bit of fettling with the xorg.conf file. I had it all working fine under 11.04 but after upgrading to 11.10 things went down hill. There is a description of the problems I had here : Upgrade To 11.10 - Issue With Quad Monitors
I am using two different ATI cards and the xorg.conf I had used to work fine. However, under 11.10 things seem a lot trickier. I have achieved varying levels of success and I did manage to get four monitors working temporarily but the system constantly froze and X would restart every 5 minutes or so.
I have since had a chat with the chaps over on #radeon on irc.freenode.net and they have assured me that my config is correct (which would seem to be correct as it worked under previous versions). They suspect a bug in Gnome or X is causing the problems.
Anyway, long story short. If I was in your position I would not expect even two identical cards to work by simply plugging them in and it all magically works. Expect to have to put a bit of work in getting your X configuration perfect manually. It is quite possible that there is a configuration out there that will work out of the box but until you have seen someone do it, plug the cards in and boot up with all four monitors on, be prepared for a bit of a hard time...
For what i know the only one brand that produce that kind of VGA is Matrox http://www.matrox.com/en/ the main difference is that their products are simply focused on CAD and multi-monitor support from years to now, but this is a really professional solution.
Regarding the consumer market ATI has a better support than Nvidia to multi monitor with it's own Eyeifinity technology.
I have done this, and I believe as long as they're both by the same vendor they should work. For me I used an old 8 series Nvidia card with a newer 400 series card... Through the Nvidia Settings it recognized both cards, However, I had to run a separate X server on each card but was able to have a desktop on all 4 screens.