I can recommend
Blue Nile.
They have excellent choices and selection, not to mention fantastic customer service (ie: you can return a ring without any problems at all--just don't ask me how I know

).
While you may be able to find slightly lower prices, generally theirs are pretty good.
With a diamond/engagement ring you get:
A Nice Rosewood box, Free FedEx Shipping, and a pretty good (though inflated as always) evaluation/appraisl.
I bought two engagement rings (same person involved) and 2 platinum bands from them (year apart from engagement to bands) and will buy from them again if I ever need to.
~AJ
PS If you were looking for a round, I could provide a ton of info/specs/details...but marquise isn't my (or her) style
