am with MB for the keeping it above 250 general rule. For larger stuff, lower is definately acceptable.
some friends of mine who have a photo lab and have added a 9000 series Epson printer that prints to i think 44" wide, regularly print big prints (over 20x24) at 200dpi with excellent results. Makes sense, as busshelter type posters etc are printed at a much lower resolution, a billboard even less.
top quality original files a must of course, and then you get into the area of what software is doing the extrapolation if needed. High end printer "RIP" software is super expensive and super good.