I know this will seem un-romantic, but I would go to at least one jewelry store with your intended to find out what her tastes are. Or perhaps you know a close friend of hers or her sister, and they might be able to advise you.
I disagree with the size-over-quality position. Yes, the quality does add to how impressive the ring is. And the size of the diamond doesn't matter so much anymore because most settings have little diamonds surrounding the main diamond, and so even a small diamond can be very impressive.
Round settings are still the most popular. You can decide whether that's a good thing (I'm doing what most guys do) or a bad thing (what I'm doing is too conventional).
Note also that some jewelry companies can find you interest-free financing for six months or so. That's a way to spread out cost without paying finance charges.
I disagree with the size-over-quality position. Yes, the quality does add to how impressive the ring is. And the size of the diamond doesn't matter so much anymore because most settings have little diamonds surrounding the main diamond, and so even a small diamond can be very impressive.
Round settings are still the most popular. You can decide whether that's a good thing (I'm doing what most guys do) or a bad thing (what I'm doing is too conventional).
Note also that some jewelry companies can find you interest-free financing for six months or so. That's a way to spread out cost without paying finance charges.