Katherine Heigl has been everywhere lately. Grey's Anatomy. Knocked Up, and now 27 Dresses. I cannot wait to see this movie and she looked stunning at the premiere. Her look is extremely expensive in her Bill Blass dress and Jimmy Choo heels. A similar looks would look great for a dance or formal event.
I figure splurge on the dress and save on everything else. It is the most noticeable thing you'll be wearing anyway. Either one of these BCBGMAXAZRIA dresses would look great and keep all eyes on you. Both via http://www.nordstrom.com/
These shoes form Aldo come in a variety of colors, so you can either add a little pop to your outfit or keep it classy in a metallic or neutral color.
For accessories take things from your own wardrobe or head over to Claire's and get some cheap, simple pieces. A sophisticated clutch will add a great touch to the outfit and have you looking better than everyone.