Well, it all depends on what you are willing to do. If you plan on door speakers and a sub anyhow, go for the door speakers now and then get the sub later. You could also go for separates with mids in the doors and tweeters in the dash. My solution is to use the mounts GM used in the Regals with the "Concert Sound II" setup which put 5.25 in woofers in the doors and 1 in tweeters in the dash. You could also get some 6x9 boxes or truck speakers to put behind the seat, depending on how much room there is. If all you decide to do is the door speakers and sub, consider getting a 4 channel amp, and bridging 2 channels into one sub channel. This way, if you get a head unit with a subwoofer gain control, you can vary the output of the sub without affecting the equalization of the other speakers. This is similar to how I run the stereo in my small pickup. The difference being that my truck has only one 2 channel amp running the 2 10in subs, and the door speakers run off the deck, and all of the fader is set to full front. I have 2 6.5 in coaxials in the doors, and the 2 factory high mount tweeters as well. It actually sounds really good. My deck is a mid grade JVC with a equalizer and sub gain built in. I also run an iPod off of it using a cheap cable I got on Ebay for $20 or so. Ebay is a good place to look for new speakers too, as many vendors sell closeouts and leftovers that are new in box. Also, consider going to Wal Mart and buying a Pioneer IMPP 10 in sub then building your own box. The sub is only around $60, and they sound pretty good too. If you are into booty music, it probably won't be enough, but for anything else it would be just fine.
Here's an auction on some nice speakers that are an Ebay closeout. For $110, they come with crossovers, and separate 5.25in woofers with 19mm tweeters.
http://cgi.ebay.com/NEW-MB-QUART-RC...ryZ32818QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I am not pushing those in particular, but it is an example of what is out there. I will be buying my speakers on Ebay when the time comes as I can't beat the prices! It's easily 1/2 to 1/3 the price of similar items at Best Buy or Circuit City, let alone local stereo shops. I buy decks locally though, as they tend to be more likely to need service, which is hard to get from an e-mail address.
. but you did bring up a good point, with the boxes behing the seat. i'd just have to find a way to connect them over the hump so they dont slide around. thanx for the ideas.
