have a look at this if you can get past teh geeky stuff
One important paragraph reads as follows:
"The megapixel myth was started by camera makers and swallowed hook, line and sinker by camera measurebators. Camera makers use the number of megapixels a camera has to hoodwink you into thinking it has something to do with camera quality. They use it because even a tiny linear resolution increase results in a huge total pixel increase, since the total pixel count varies as the total area of the image, which varies as the square of the linear resolution. In other words, an almost invisible 40% increase in the number of pixels in any one direction results in a doubling of the total number of pixels in the image. Therefore camera makers can always brag about how much better this week's camera is, with even negligible improvements.
This gimmick is used by salespeople and manufacturers to you feel as if your current camera is inadequate and needs to be replaced even if the new cameras each year are only slightly better.
One needs about a doubling of linear resolution or film size to make an obvious improvement. This is the same as a quadrupling of megapixels. A simple doubling of megapixels, even if all else remained the same, is very subtle. The factors that matter, like color and sharpening algorithms, are far more significant."
He goes on to compare some pics at different resolutions to prove the point.
He offers a calculation to show the maximum print size recommended to retain sharness:
"Long print dimension in inches = 4 x (square root of megapixels)"
For example, for a four megapixel camera the square root of four is two. Two times four is eight. Thus the biggest print you can make without losing sharpness compared to film at normal viewing distances is is 6 x 8." From a sixteen MP camera likewise you could go 12 x 16." Of course you can print bigger, just you won't have the sharpness of film. Also few people are able to get all the sharpness of which film is capable, making this harder to compare.
Based on this
3megapixel - max long side 6.9" (i.e. 6*4 print will be fine)
4megapixel - max long side size 8"
5megapixel - max long side size 8.9"
I would stick to higher resolution if you can, but if you want to drop the resolution to get more pics on you probably won't notice dropping to 4 or 3 (according to that article.
Why don't you try it before you go? Take a pic of the same thing at diff resolutions and see if you can tell.