A typical upscale affair usually includes a sit down dinner, but depending on the size of your party, you may not have enough seating for all of your guests. If that's the case, heavy hors d'oeuvres served buffet style is perfectly acceptable.
As far as types of foods, if you want to pull out all the stops, you can impress your guests with the likes of caviar and all the trimmings. If that's a little over the top for even your most elegant friends, a fancy shrimp cocktail will suffice.
Regardless of how you serve it, the food should be high quality, substantial, and served on proper plates instead of plastic ware. If your budget allows, we highly recommend that you get a caterer so that you can just be the hostess.