If you are going for a diamond ring then you should really consider ordering online from http://www.bluenile.ca/
Same quality diamonds as a store such as People's, but you'll pay a lot less.
I took my ex-wife's ring back as part of the agreement. Screw her!
Then, I thought, okay, if I'm ever thinking about asking someone to marry me again. Look at those two rings and go see an escort!
If I do get married, well, I'll just say sweetheart I picked these rings out just for you! Screw her too. Save the money for my bachelor and divorce parties!
I took my ex-wife's ring back as part of the agreement. Screw her!
Walmart!!!!
I find an unmounted diamond looks better before it is mounted in a ring. So I advise looking at diamond rings with the diamond already set so you can see if the sparkle you like is there.
Two words....cubic zirconia...indistinguishable from natural diamond to the average citizen.
Even better, synthetic moissanite, it's stronger and has a higher luste and sharper facets which make them actually better looking that diamonds.
http://en.wikipedia.org/wiki/Moissanite
There are good deals to be had on ebay, but I would lurk and research for long time before bidding.
The engagement ring is a trinket that she will show off for about a month after you pop the question.
It won't fit during pregnancy, and after birth, she probably won't wear it for fear of scratching the baby.
The future missus may see things differently and making her happy is an important consideration.
Good luck.