You don't need to buy an HDTV for the digital switchover because it is not going to be an 'HD' switchover, just a 'digital' switchover. This doesn't mean that shows will all be in HD, so you don't need to go out and buy a brand new HDTV. If you have a television or HDTV built after March 1st, 2007, then you are fine, you won't need the digital converter box. As of March 1, 2007, every television manufactured is required to have a digital tuner built-in. What that means is every television sold in stores today is digital television ready, no converter box is needed. If you have cable, you are fine. The signal will switch to digital automatically and you won't need a box, unless your television is old.
Now, you may need a box if you want all the HD channels that your cable service provides. The box they will give you is not a converter box, but really a descrambler box so you can get those HD channels.