I just purchased my second engagement ring. She doesn't even have it yet.
My thoughts:
1) Buy online. Whiteflash, Bluenile, or any of the biggies. You'll save at least 35% compared to Zales through Tiffanys. Do some research and you wont get ripped off. You should expect a certificate and should be offered an independent appraisal.
2) Shop with her? Studies show men pay less when the woman is involved (obviously not a controlled study, but valid if the population is large enough). If she talks you down a notch so you can save for a sofa, let her.
3) Drop down a couple of levels for color and clarity. An untrained eye can't tell the difference, especially when mounted (the ring, you idiot). Size matters, for this and the other thing, too.
4) Chances are, you really wont care that much about the expense until/unless you get divorced (then you'll immediately feel like a fool for wasting that much cash.) But try to pay in full. It's harder to feel as good about the spend when you're still cutting a monthly check 7 months after the wedding.
5) Don't believe her lies that she doesn't care about size. The most modest girl in the world still appreciates feeling special when you suffer a bit for her (even if's it's for her friend's sake).
6) Don't go by month-salary rules. There's too much personal variation that doesn't align with stone prices. Talk to your pals, get a sense for what her friends (sister?) have, how you shape up with that scene, and understand YOUR and YOUR COMBINED financial health, and feel good about whatever you decide. You may want to engage her in a discussion about your budget then go a little above that to make it seem extra special.
7) I don't know of any girls who reasearch the stone quality afterwards. It's pretty safe that you can get it anywhere and she'll never ask about (or understand) the 4 C's (let alone fluorescence or girdle angles)
Hope this helps! Congrats!