New TV purchase, is it an upgrade?

Me and my fiance just purchased a Sony 55" Grand Wega (KDF55E2000). It supports 1080i but has a native resolution of 1280x720. We were at a few stores and the picture looks unbelievable especially for what we paid but my only concern is right now I have a 1080i tv (with no 720p support.) My current tv is a 42" Mitsubishi RPT which supports 480i, 480p, and 1080i. The new tv supports 1080i but it downconverts it to 1280x820. Is this an upgrade or am I going backwards?

Don't know its being delivered on wednesday but I figured I would ask an opinion before it arrived. In the store it looked incredible next to a mitsu 1080p unit but I also know that what you see in the store isn't always what you get at home. The picture definately looked sharper but I'm used to looking at E* HDTV and upconverted dvds so what they had running is probably better than what I'm seeing.