My fiance talked about marriega before I was ready to, so when we were looking around in a jewelry store, and I picked out a pretty engagement ring setting, that was pretty much a go-ahead to propose when he was ready. He chose the diamond, and bought it when he was ready.
A lot of jewelry stores will sell you the diamond itself, and set it in prongs on a simple band, so that when you get engaged, you have it on a ring, but then you can come into the store and put your diamond in the setting you want. That way you arent stuck with something you hate, and he isn't put under too much pressure to choose.
Also, if you decide on a ring that doesn't fit with a band, I have seen a lot owomen use their engagemnt ring as the band, and only have one, not two rings. Or if you have enough money, you can have a band custom made to work with the ring.
Hope that helps!