Just throw in my idea although I am not experience in this area.
In general 72DPI (Dot per inche) is good enough for general quality. A 15" LCD monitor with 1024x768 is having about 85 DPI and if you are happy with the display, that is good enough.
If I based on the calculation, 3m x 2m will requires 8845 x 5896 pixels to have resonable quality at close up view, this translate into 52Mega pixels. But then who will view such a big posters at 1 foot distance?
If you are hanging the poster on top of 10 storey building along highway to catch attention of drivers, then 2 mega pixels is sufficient.
A projector with 1024x768 resolution is good enough to project picture up to 2m x 1.5m on the screen for viewer sitting at 10 feets away.
The answer depends on your contents and how is it going to be viewed.
Get someone to shoot a 4x5 positive, then have that drum scanned. Expencive? yes, but it gives you decent quality. Now, something to remember is most billboards are UNDER 100 Megabytes saved as TIFF, so just remember viewing distance.
best thing is to save your file and
Image-->Image Size, set to how you want it
View--> Print size and thats how the print will look.
Thats what you need to do in photoshop to see how anything will look.