As Scott said, any brand name is going to be a huge improvement over stock, however if you're not going to run amps and upgrade your deck, you don't need to spend lots of money on speakers.
I picked up some Pioneers (no longer made, guess they were getting rid of their stock) last summer for $50 for fronts and $50 for rears (reg $100 each pair). They sound a thousand times better than the old stock speakers.
One thing that I think really helped, was buying
these adapter plates that let you install 5.25" speakers in the front. You can also (sometimes) find 6.5" speakers that won't require much work to fit into the sound pods. (Unfortunately my 6.5's required a little finessing with tin snips and a dremel to get them to fit)
Good luck!
Also...I upgraded because I thought one of the rear speakers was blown out, turns out the map light covers rattle in the sound pods sometimes. So I have a full set of used factory speakers if you want them instead.