Future Attachment For My Normal TV?

First, this has more than likely already been asked, and I apologize for repeating the question, but in doing the search, I probably didn’t know what to enter into the field…

Okay, so in 2-3 years, our tv’s will all be digital? And those of us who decide not to spend the extra bucks will be required to purchase an attachment for our older sets to convert?

How much is this thing gonna cost me, and will it improve the picture or just convert the digital signal to one my set can interpret and show me in the older style?

Rather than go into detail, a link would suffice.

Thanks

Quasi