First of all, that is great that you and your boyfriend are discussing marriage!
Now to the rings. We did tons of research when looking for an ring for me. We went to several jewellers from stores in shopping centres to Tiffany . We ended up getting my ring from a jeweller in a different city. It was worth the effort.
I would suggest that you go shopping either alone (or maybe with a friend who can keep a secret!) or together, look at rings and try them on. Online is great to get an idea as to what is out there but I personally believe that you need to try on some rings to get an idea of what you like. Once you have tried some rings on and you have an idea as to what you like (metal, diamond shape etc) by all means check out the online sites. When looking at online sites, always compare like for like diamonds - compare the price of diamonds of similare colour, cut, clarity, and carat.
I have not had any experience with Platin-Ice but maybe some other member might be able to give you some insight regarding their product and service.