Op, If you don't wanna spend a lot of $$$ or simply can't, you can get diamond engagement rings for under a $100. They won't be massive rocks or anything but it's better that a cubic z. I had a so-called "quality" cubic z and as others said, they get yucky over time. Here's some rings to check out:

Under $100:

[link to www.helzberg.com]

[link to www.helzberg.com]

Not sure if you wanna spend a grand or not but the one below is nice. It's got 1 CT worth of stones in white gold for $999. The "center stone" gives the illusion of a big engagement ring but is actually made of many small diamonds as opposed to one large rock ring that cost $10k:

[link to www.helzberg.com]

What ever you do, don't go deep in debt for a fancy ring. You could take the 5 to 10k and use it towards a down payment on a house or to get you and her out of debt.