I don't have time to read all the posts right now, but I've worked selling diamonds for about a year now and I have a few opinions on the general buying process.
First, buy from a nationwide company with a FREE diamond warranty. I work at Jared: The Galleria of Jewelery (yes that's how we have to say it) and their prices are very fair and the warranty is great. If the diamond chips cracks or falls out we replace it for free, and that has no strings attached. We do ask to inspect the ring twice a year, but that isn't too much of an inconvenience for most people.
Second, buy what you can afford. Don't go into a huge amount of debt for a ring. Finance it through the store if possible (we have in house financing), but also check your bank/credit union for a signature loan.
Third, the 4 C's definitely are important, but don't get caught up in them. The fact is grading of a diamond is incredibly subjective and the different companies which grade the diamonds will grade most diamonds differently. IGI, AGS, and GIA are probably the 3 companies to trust while AGL, EGL, etc. aren't the greatest. I bought based on color for the sole reason that anyone can see a yellow diamond, but not everyone can see an inclusion in an I1 diamond. I'm assuming this has been explained, but if it isn't and you wanna know about it PM me. Also, look for ideal cuts certified by AGS and GIA. They will cost you more, but the brilliance is well worth it.
Finally, go into a store your comfortable with. For me Jared is great because it is very roomy, it has a ton of settings and loose stones, and everything is priced as marked. There is no negotiating price and, at least in Utah, the prices are lower than stores with similar warranties in the area.
There's my .02 and a slightly shameless plug for my employer.
p.s. If your in the Salt Lake area and looking at getting married come see me