It is probably the best upgrade for the very smallest amount of money. The speakers absolutely suck. The stock head untit will power some better speakers and will definitely sound better.

If you are willing to mod the front speaker cage a bit, you can fit 6.5" speakers up front. I have 6.5" Infinity Reference 2-ways in the front and they sound great!!! I shoved some Infinity 6x9's in the rear (modifying the rear speaker cage) and blew one of them, so I'm going to 6.5"s very soon.

I'd say, go for the 5 1/2's if you don't want to mod the front cage... it will sound better than stock for sure!