For print on demand 300dpi is considered adequate for graphics, except when it was expanded upward from a lower raster dpi resolution. A submittal with images less than 300dpi is rejected by CreateSpace's automated error detection during pdf-book submittals, for example. So anything below 300dpi is considered visibly less sharp by the book industry.
If you know specific images print size are going to be reduced by half (so that four can be fitted on a single page) then 150dpi for the originals should be a minimum adequate resolution, given the 300dpi standard, and given that you only reduce the dimension, and not the number of pixels. If you choose 150dpi you should double check the method you use to reduce the print size of any graphics so that the final graphics come out to at least 300dpi.
I do hear of folks using 600dpi, because they can, but I can't visually see the difference between 300dpi and 600dpi myself. [note that .eps is a vector graphic format that may also contain raster.]