two 15's wouldnt be hard. but you have to be careful of the extra power you are adding to the cars electrical system. a stock alternator can only handle so much. anything at around 1000 watts and under i would say is safe. but i would upgrade the battery and do the 'big 3' nonetheless. also...start small. if you are shy to install it yourself, then yeah best buy/circuit city may be the best. then you can see how it's done and go from there and learn how to install yourself. i say start small cuz chances are you will mess with things...wiring, gain sets, etc...and will blow somethin up. like just about everyone when they start out. at least till they start to learn the more technical side of things (ohms, ohm loads, different wiring options with the given voice coils). so it's better off to ruin something small and cheap than something big and expensive. work your way up to the big and expensive things and you wont have to worry about messing them up.
btw...did you have any certain 15's in mind?