Yep - call the printer and ask THEM - believe it or not, sometimes you can even go down to 100dpi!

Often, some printing equipment (like a Lightjet or a Lambda) will do a slight version of "filling in the blanks" and smooth out stepping/jaggies. This saves not only YOU for having to create a huge file, but it saves THEIR SERVER and equipment from having to HANDLE it.

You'd be amazed at how low you can go and it still look great - especially the larger you go (billboards are often only 9, 12, or 36dpi!).

Hope this helps-
-Lew ;-)