If you want to go for a deck and just want a basic setup,
http://www.ikesound.com/product-product_id/1814
Nice one, isn't it? =) Rotary volume control and 2x RCA Preouts. $135! It will improve your sound quality dramatically. Pair that up with two or four of these:
http://www.ikesound.com/product-product_id/1344
$55 a pair. Remember, you can probably find all of this on Ebay for cheaper. But for $245 total, you can have a deck and four speakers that will cause people to laugh at the Mach system which costs a lot more.
Now of course you'll need the face plate and all that jazz. If you are really insistent upon the stock look, replace the deck suggestion with this:
Puts out just a bit more wattage than a deck, you can attach this to your stock amp and wire up the speakers for a fairly decent sound. It lists for $129 at ikesound.com, but you can find it much cheaper on Ebay.
Edit - And just to let you know there the deck is a Pioneer DEH-P2600, the speakers are Infinity Reference 5x7", and the amp is a Kenwood KAC-6401