I recently got engaged and I always wanted a princess cut but I got a round brilliant and it's absolutely stunning mA! It has tiny diamonds along the band, like the pic sumorani posted earlier. It's platinum and I would actually say platinum is better for wearing all the time because white gold is regular gold covered with another metal (chromium I think but I'm not sure) and it wears off over time so you'll need to go in every few years to maintain the shine and look.
I think the rule is three months salary but don't know how many people follow that rule since rings are insanely expensive. We all want a big diamond but even if it's big you don't want it to have flaws or not be clear, if you're not going for the pretty exotic colors that is.