I personally would get a new screen for your main, and run the others on your existing screen. (says the man with 5 screens atm :p)
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I travel a lot for work, so I use 2 setups. Dual Monitors at home (preferred) and 5 on my 17" laptop screen on the road. For the laptop I use a 1920x1080 main region (full resolution is 1920x1200) with the slaves at the top left using 304x171.
I'd recommend getting another monitor if its an option, but keeping them all on 1 really isn't bad. Just be sure to select "Slave regions always on top" for some nice PiP if you go that route.
I find Jamba more helpful then looking at the other screens (follow/health/mana bags full warnings) I have two monitors but as mentioned above, rarely look at the slaves.
ISBoxer does have a single screen config for 5 toons... you should be fine.
I picked up a 25inch INC monitor from bestbuy and 5 boxed for a while. I decided to go get another one and its a lot better of course but one monitor wasnt bad at all. I got the INC's on sale too. I paid like 240 for each one.