The Q6600 will handle 5 (I've run 5 on an E6600 and it's playable), but it's your call if you'd rather just spend the extra money now for a Q9450. I would.

I also run my main in a 1680x1050 window on my 1920x1200 24" monitor, and it's decent having 4 alts on a 1680x1050 screen, although I plan on putting 4 on another 24" sometime soon. (4 in a 22" 1680x1050 is just a little too small to continue like this much longer.)

For cases, it's more of a personal preference than anything. I love my Antec P182 - quiet, cools well, and solid. The Antec 900 and 1200 are a bit more flashy, but have excellent cooling at the cost of noise.

For motherboards, there are a bunch of X38/P45/X48 boards that are very good - I like the DFI boards right now. Or, you could go 750i/780i/790i if you want the possibility of SLI in the future. I don't care much for SLI, but I *do* recommend 1 videocard per monitor in a non-SLI config. The big hit on fps for me is splitting my 8800GTX over two monitors.. I'll probably add an 8800GT when I upgrade my CPU/mobo/ram so I can dedicate one card per display. Motherboard choice is all about how much you want to spend, if you're going to overclock, if you are going to use a RAID array, and if you'd like to go SLI/Crossfire in the future. Can't narrow it down without those options..