That is so strange, as I been looking at the Apple Store online site for over one month now and it always shown a 750 GB hard drive option that could have been picked in the configure menu, along with a 3 GB option for ram though very expensive thru Apple. Also there is a 256 MB option video card so to me this is nothing new.
I wanted to add that this is for the 24 inch imac model only. The 20 inch shows only 500Gb Hard drive option. So you may be right and I thought you were talking about the larger version ( the 24 inch model) .