You can find a pretty good list of cards supporting Eyefinity 5 or 6 at http://www.wsgf.org/forums/viewtopic.php?f=66&t=24334 I used to use the AMD 5870 Eyefinity 6 2GB (Reference), and am currently using a Asus Matrix HD7970-P-3GD5 3GB.

I mostly single box on a 5x1P (5400x1920) + 1P Portrait layout. I have done a bit of very casual 2-3 boxing, and the biggest issue (boxing versus not) I've found is sheer the # of pixels you end up rendering displaying. (5 @ 5x resolution is a lot of pixels). In all modes, the dividers are also somewhat annoying (especially for text), even though I don't bezel adjust and hide anything behind them. Also, even the smallest UI scale is too big for me. Thankfully, MoveAnything can help alleviate both of those.

I regularly debate switching to fewer and larger monitors. Now that AMD's drivers support mixed mode, my current consideration is a 2560x1600 monitor in the middle, flanked by portrait 1920x1080 monitors, to have a wider center region without dividers.