It has been a few years since I bought my wifes diamond but at the time I did a lot of research. The best website I found for information was:
www.niceice.com
I went with quality over size when I bought her diamond. Most people have no clue how big a 1 ct diamond actually is, so if the diamond looks good they will assume it is larger.
From what I remember, the cut is the most important thing to consider. I got a "hearts and arrows" cut and the sparkle is amazing. It sparkles in colour, not just white. I bought from Wink at:
www.diamondsandgems.com
It was a good experience and I got a very good price on it.
Just my thoughts.