I think you should take her shopping and let her try a few rings on that she likes. Don't buy it that day. Go back another day by yourself and pick one of the rings that she loved.
The ring you have chosen is nice but the metal takes away from the diamond. Also, are you planning on getting a wedding band made to go with the ring?? If so, you might want to think ahead and get a ring that can easily have a band made to go with it. Not all engagement rings are made to have a wedding band.
My engagement ring is beautiful and I wanted my wedding band to have a few extra diamonds to go with it. The custom jeweler talked us out of having a band made and I ended up getting a whole new ring for my wedding ring. I am not complaining...its 3 karats of diamonds.
Shop around. The big retail places will rip you off compared to some of the mom and pop jewelry stores.
Good luck and also pick out your ring while your out shopping with your GF.
Keep us posted.