Personally, I wouldn't buy a ring at Amazon.com but you have to live within your means. And if an Amazon ring is the best ring you can afford, then that's the ring you go with. However, whatever you do, just opt for cut, clarity and color over carat.
Have you checked Overstock.com? If you're going to buy via a cheaper, online merchant, I recommend buying through them over Amazon. They actually have some really nice diamonds. I bought some diamond stud earrings on there a few months ago and was blown away when I received them. I had them appraised at 40% over what I paid for them. Much better than what I was expecting to receive. And they have a HUGE selection of engagement rings. Just something to consider over Amazon.com. And you can usually find a coupon code for Overstock to get an additional percentage off (low shipping charges too).