I think you need to work out a few specifics first.
For example; how much space will you be offering to each user and how do you define popular? I think if it's free hosting you probably don't want to be offering over 100mb of webspace and perhaps 500-600MB bandwidth.
Using those packages you could host approximately 60 sites on the standard linux reseller package at eUKHost (for example).
Even though this isn't a great deal, it's plenty for a simple brochure style
website hosting and I'm sure if you offer a few e-mail addresses, PHP support and perhaps a mySQL database with your packages people will be quite happy with that.