If you're going to buy online, go with Infinity anythings (Kappas, Reference, they're all solid). That way no matter what you'll buy, it'll never suck.
If you're buying local, I would hunt down a sale at a local car stereo shop. Most of them will let you haggle on the price. Although Sony Xplod decks are pretty cool, I wouldn't recommend their speakers, as they don't deliver adequate bass, almost requiring a subwoofer. And don't buy into the more is better philosophy; a 3-way speaker is not always better than a 2-way speaker.
Lastly, try to pick up speakers with angled tweets. Most Infinity's do have angled tweets.