It is surprising how much you can blow up some images, but not everything works the same. It has more to do with the quality of the image, and the viewing distance, rather than the resolution.
The 'best' resolution for photo prints is from 240dpi to 300 dpi (dots per inch), which would allow you to blow your image up to 8 x 10 for a photo-comp type image. However if you wanted a poster, say A2 size (594x420mm), then you'd drop to about 100 dpi, which with a nice bright pic would make a great poster -- just a bit less impressive up close.
In my experience (15 years in a digital print shop) people are always happy with their giant prints, even if they end up printed at 15 dots per inch:)
Here's a
chart from Digitalmax, where I get my prints. It shows the minimum requirements for each print size.