Not all 720p are made the same.

Are they encoded in H264 or XVID? What is the file size?

Generally, i would say it needs to be encode in H264 and size around 4GB+ (Some 720p can go to 6GB to 8GB) to be of good quality, and the source must be BLUE RAY (not recorded from HDTV channels). There are also 2GB "720p" rips, which is definitely inferior. And rips in XVID have inferior quality to H264...even when the rips in XVID have a bigger file size. i have a 2GB H264 rip which is way superior to a 4GB XVID rip.(same movie).