I have the same monitor, you can see what I've been using to this point in my sig. The breakdown is one 2560x1000 window and 4 640x600 at the bottom. This is what I settled on as the best tradeoff between main window size and being able to do anything reasonable on the clones.

I recently (on Rob's suggestion), tried out the 4 little windows on the right to maintain the same aspect ratio for those PiP moments. I definitely liked that part of it, but I'm still on the fence about the overall effect (mainly the "scrunched" horizontal feel of the main window, possibly because I'm so used to the mega-wide one I've had to this point ). I will say that while the non-consistent aspect ratio thing looks like crap, it's totally playable to me, especially considering that I'm only doing so long enough to get my main back up and going. My opinions could change if I spent a lot of time switched (like after a main death in a bg, for instance).

And to answer Xzin's question, I really don't care much what the clone windows look like as long as I can read them (barely) and see what's going on. In any situation where I need more than a trivial interaction with that toon, I PiP and use the main window.