I second the Crutchfield site. Even if you don't plan on buying from them, they are a great place to research components and have very good sorting and filter tools to narrow your options. The radio is a single DIN, so there are a lot of options. They also have wiring adapter kits for the radio, so if your wiring hasn't already been butchered, these adapters make the radio an almost plug and play job by plugging right into the existing harness.

As has been said, there are only two speakers as standard, one in the left and one in the right side of the lower dash. I have not replaced the speakers so can't help you on getting them out other than the speaker cover comes off and you take them out from the front of the dash. As for the radio, I can tell you what I put in my car last year. It was the Pioneer DEH-X9600BHS. Told myself that I would never by another Pioneer after a button failed on the almost new radio the previous owner installed, but I couldn't pass up the radio due to all the features it has. Everything from CD player (hidden behind the removable faceplate) to Bluetooth to handsfree calling to dual USB inputs, and even Sirius if you use that. Probably not the cheapest on the market (I paid around $185 and you must shop around for the best deal), but I liked it enough to buy another for another of my cars. But like I said, if you don't like Pioneer or want something cheaper, there are a ton of single DIN options.

If you want to greatly improve your sound, might want to think about adding rear speakers. The fronts are pretty small and never designed for spectacular sound. The previous owner of my car added a pair of rear speakers mounted in separate enclosures that just sit on the package shelf. I liked that he did not cut holes in the shelf or alter the car to add them, but have seen pictures of others who mount them in the shelf. The sound in my car is very good, something I attribute to those rear 6x9 speakers.