Bear in mind that the higher the resolution, the larger the file size and depending on your HD size and number of pictures that could be an issue.
What are you planning on doing with your pics? Are you putting them into the Mac just to have a reference to them? Are you planning on only ever using them at their current size? Do you intend trying to enlarge them/edit them at any point? Reference only to look at on screen, you'll get away with a low res, at current size, you'd be looking at scanning them in around 300ppi, if you plan on enlarging them etc you might be looking higher.
Things to consider when buying a scanner - are you just planning on scanning prints or do you want to scan negatives/slides too? Do you want a feeder so that you can put a pile of prints and and leave it to it?
I'm just about to get a scanner to do this myself so will be interested in seeing suggestions. Currently, since I can't see myself scanning in ALL my photos - just the highlights and not from the negatives, I'm planning on just getting the Canon Lide-35 which is £49 at the moment. It does store on its side, doesn't have a feeder, is powered by the USB and seems to get good enough reviews that I'll be happy. After all, it's not like I plan on dumping all my current images should I decide I desperately need a higher res in the future.