Having just purchased a diamond for my g/f. I was origionally focused on the exact same type of diamonds you have listed here from reading a few posts that my g/f showed me on this bbs. However after looking at quite a few diamonds and listening to "the diamond talk" at many locations, I would have to suggest that you might look at something a little different. Once you select a carat size, roughly 50% of the cost of the diamond is due to the "cut", 30% is from "clarity", and 20% is from "color". If you don't mind me stereotyping most girls, bigger = better. So spend most of your $ to get the largest size and best cut you can. The problem with "cut" is that there is no clear rating system, like clarity or color, so everyone seems to focus most on these two areas. Without using a 10x magnifier you will not see the differences between a clarity rating of VVS and VS, and it would be very very difficult to even see the differences between the VVS, VS, or SI1 with the nakid eye. As for color, if you are going to mount the diamond in a gold band, you will never notice that the diamond is slightly colored (Color K-N atleast). If its white gold or platinum, yes, you need a diamond with less color, but anything above I or J would be fine.
My suggestion is go look at loose diamonds of different carats, cuts, and clarity to learn what is most important to you. But, in my humble opinion, what you (and most of your friends) are going to instantly is size and how much it sparkles, aka "cut".