I suggest looking for a Freeview HD box, even if you've not got an HD-ready TV set. That's because some new SD channels are now being added to HD multiplexes, meaning that you must have HD equipment to receive them.
For example, Talking Pictures is an SD broadcaster (on Freeview Channel 81) but I can't receive it with an SD Freeview TV. Similarly, Rishtey Europe (for Bollywood fans, like me) is transmitted in SD on Freeview Channel 77 but again (irritatingly) I can't receive it because its been placed on an HD multiplex. More new channels are likely to follow the same practice (because the HD multiplexes are the ones with spare capacity, even if the broadcasters are only using SD).