there are a few things to consider...personally, i prefer the dual setup, while the single monitor looks "cooler" & more expensive IMHO...
If you always have multiple windows open, it's more advantageous to have a dual display since it's easier/better to organize things that way...i dunno...It's nice to have Quake loaded on 1 monitor and work loaded on another, just MHO anyway...
Here are some numbers to play with...
assuming the 17in is 13in * 11in, if u get 2, the effective size is 'bout 28in...