Very nice setup. I'm only using one 27" at the moment, but I have a spare 19" that I'm trying to find room for. I've ran 3 monitors like your base setup there, but I have NO idea what I would use the other two for. But hey, the more space the better, right?
Question- I see your tower at the bottom left, but are the other two black boxes servers? Or are they part of the desk itself?
I use a GTX470 for my main monitors, and an old 8800 GT for the secondaries. If you don't need to SLI, you can use as many unmatched video cards (as long as they don't conflict with the other cards' drivers) as you want. I don't play a lot of games anymore, so my video card is a little out of date...
Nah, I think the resolution limit of the 470 is higher but I never hit the limit - I just run 1920 x 1080 on all the screens(except the smaller 1280 x 1024 at the top). Incompatibility shouldn't be an issue, since NVIDIA went with a unified driver framework since the (6xxx) series(?). So you should be able to run the 9800 GT fine, just install drivers for the 560. However, whichever card a screen is connected to determines which card renders it- so you want to make your primary Photoshop monitor connected to the 560ti. If anything goes wrong you can always turn off the computer and disconnect it anyway. Make it a weekend project?
I buy all my monitor mounts from monoprice (.com). They're the only place I could find arm mounts for around $20. I paid a little more for the dual vertical mount though. Keep in mind that if you set them up correctly they're pretty stable, but don't expect them to be nearly as full-featured as the $70+ mounts.
Yeah, having a virtual pegboard for references is really great! It's nice to have a window full of textures that you can drag into Photoshop to bash, too.