Yeah it sucks that Crutchfield only has a few brick and mortar stores. I happened to be in a town for a wedding that had one -- immensely helpful to be able to compare some of the different radios and subs in person.
As far as factory speakers go -- yeah you'll probably want to change them, even if you don't do an amp. Just about any aftermarket speaker is going to sound clearer and fuller than these tiny little things:
Compare that to a pair of reasonably priced component speakers:
You'll hear a world of difference just w/ the deck and speakers. But an amp really is icing on the cake -- and you don't need a huge one. I put a 75Wx4 RMS amp my DD Focus -- the sound is so much fuller than my Mustang. The only thing it needs is a sub now. I keep messing w/ the EQ on my GT but I can never get it to sound as good as the setup in my Focus. The component speakers give you clear highs, the factory subs still give thumping lows, but its the midrange that seems to be coming up flat. I'm really tempted to buy another amp for the Mustang
But I only drive it a day or two a week.