If you expect that a large part of your customer base is going to be "lite users", I think you could probably do well if you halved each of your plans, or close to it. There are a lot of websites that won't go over 200-250mb of space, especially personal websites.
I mention this because you don't want to sell a bunch of 500mb plans if a majority of those plans used 200mb or less... that's a lot of wasted MB that could be used for new accounts.