the concensus around here is, you dont need anything above 600 dpi for photographs and i tend to agree. heck, i only use a 300 and seems fine to me.
be just a bit wary of some of those 1800 x 1800 type ads. those are the same thing you see in digital cameras where they say 24x (digital, not optical). basically, all that amounts to is the same thing as zooming in in photoshop or
psp. now, that's not to say a scanner couldnt scan at 1800 x 1800, but read the fine print first
the thing with the built in filtrations of scanners is, they vary quite a bit. and, they can cause 'lies' in the image. now, all i mean by lies here is alteration. that's why we like base, clean, raw scans. we dont want the alterations or artifacting that some do and so on. i'd point you to a couple of images here on the site where we collectively pounded our heads on the wall but it's too painful
Craig