A Few Simple Questions

Hey, I'm new to Blu-Ray quality files and whatnot and have a few simple questions. To start off, I have a 23" widescreen 1080p monitor that I'm watching all of these on.

- I've seen Blu-Ray ripped torrents labeled as 720p and others at 1080p. I was under the impression that Blu-Ray was all 1080p... so how are these being ripped to 720p? And how are either being ripped to 4-8gb? Is there a noticeable amount of loss in quality from the original disc because I don't seem to notice anything.

- On a monitor the size of mine, will there be any discernible difference between 720p and 1080p? For example, if I have a movie in 720p and a release of the same comes about in 1080p, would it be worth my while to replace the 720p or would it be fruitless?

Thanks anyone and everyone. I've been reading these forums for quite a while and they've always been useful. There's a lot of good information on here and it's cool to be able to come to a place like this and find so many people with knowledge on these subjects.