Blue Ray Quality with a 1080i / 720p HDTV?

As I understand it the new Blue Ray system is based on 1080p for its advancement in picture quality.

I have a few years old Sony 60" LCD that outputs 1080i or 720p ,but, NOT 1080p.
I should state that the picture quality with my present equipment with Hi Def sources is great.

I am sort of interested in a Blue Ray player. The Player cost seem reasonable, but I am still on the fence about the cost of the Blue Ray discs.

I have never received a straight answer as to the video improvement with my present equipment using a Blue Ray player and Blue Ray discs from the adolescences who work at the local Electronics stores.

So, here are the 2 questions...

1. Will there be any Video improvement with using a Blue Ray player and Blue Ray discs with equipment that cannot output 1080 P?

2. If there is an improvement is it 90% 50 % 25% or whatever?

Thanks,
ozzy