I shall give you my example.
My projector is 720p.
81" screen.
I use a PC to output video to the PJ.
I use ffdshow filter, which allows me to scale video to any resolution that I choose.
When I switch from 720p to 1080p output, it makes no difference to my eyes; and thats when you consider that my screen is much bigger than yours, hence any changes will be more pronounced on my screen than your smaller screen.
The key here is that my PJ and your TV have native resoultions of 720p, hence, in both cases are being scaled to a 720p resolution by the PJ (and in your case, the TV). Hence, this would explain why I cant see any difference.
In theory, if I had a a video whose resolution was 10000000x10000000, the PJ would have to downscale it to 720p once it receives the video stream, as that is its native resolution.
In summary, if you managed to get 1080i to register on your Sagem, it probably wont look any different to the 720p image you are seeing right now. And like others have said, 720p is pretty damn good. If it looks sharp on my 81" screen, it should look even sharper on your 45" screen.
From research (I was considering buying a Sagem DLP at one stage, but decided to go the PJ route as it offers better value for money), your Sagem telly (if it isnt broken), offers fantastic images on 720p feeds. I actually preferred the image quality of your Sagem to the much lauded Sony 55" SXRD Rear Projection set.