Yes, go for the biggest you may want to afford but don't let that be the only price-driver! There is so much more to it. I went with one that has very little color (colorless being the best) - that means the ring is very shiny and clear, which will get her lots of compliments. Buy her a high quality single diamond instead of spending money for additional stones or baguettes. There is a wholesaler in Chicago at www.dimendscaasi.com(I got to know him really well). He does business over the internet, check out his site, even if you don't buy from him, there is a lot of education about what to look for in a diamond. I researched for 3 months. If you want to be safe, get one that is GEA certified, you can ask your jeweler to do that, it costs 200 bucks. It is the best certification out there. Take your time and make sure that you compare apples with apples, it is very tricky! Have fun...