Is Sony's FD Trinitron WEGA XBR KV-36XBR450 an HDTV?

No, but it is an HD-Ready TV.
An HDTV displays HD signals straight from the source, such as an HD antenna.
An HD-Ready TV can display HD signals with the use of a separate converter box.
If you want just over-the-air signals of your local broadcast
stations, you will need to buy an HD antenna and a converter box.  If
you want HD stations from your cable company, they commonly supply a
converter box with their HD channel package (if your TV were an HDTV,
your cable company would likely supply something called a cable card,
which goes inside your TV).  If you want HD channels from your
satellite provider, ask someone else, because I don't know about that.

Oh, by the way, inputting KV-36XBR450 into Google's search box would
save you $2.50 getting an answer. I always wonder why people ask
certain questions rather than search Google themselves.