If you're having a tasting, you can't go wrong picking a couple of varietals and buying three or four bottles, each from a different region/appelation. You might spend a few bucks, but you'll get it back with conversation and the undying respect of your guests. Plus, it's more fun.
Try this: Buy three bottles of syrah/shiraz -- one from the Russian River Valley (maybe Arrowwood or Novy), one from the central coast (qupe is a good one) and one from Australia (two hands, as mentioned, is strong, as are mollydooker and Peter Lehman). Do the same with another varietal and everyone will have a chance to taste the difference in wines made from the same grape but grown in different regions.
Unless you have serious white wine drinkers, I'd keep it to red. Part of that is personal preference, and part is knowing how mixing the two can result in a massive headache the next day. Cabernet and syrah work well together in a tasting, but go with the syrah first and then the cab, which is less fruity and more tannic.