this belongs in the car audio section not home audio.
But I'll answer your question anyway: replace the stock speakers for sure and add an amp BEFORE adding an aftermarket receiver.
Most aftermarket head units under $300 just give you a nicer face and more bells and whistles, doing an external amp will give you a much much more dramatic improvement in SQ.
As for subs, unless you are a rap fan and want to shake windows, a single self-powered sub like the Infinity Basslink should be plenty and saves you the hassle of getting a sub amp.
why are you so sure this is a "car audio" question? i have a HOME reciever and an electronic crossover (paradigm) and a SEPERATE integrated amp driving my sub. i put it together piece by piece. NOTHING in this question says car audio to me. careful when you jump to conclusions.
regardless. to replace, or not to replace...
is there something wrong with your speakers? are they doing (or not doing) something that is driving you nuts? is there something in particular you wish they did better?
if they sound good enough to you now, then why bother? virtually every sound system can be improved, but the better one is, the smaller the improvement you'll get is.
if you want advice on how to improve your system, you'll first have to state what's wrong with what you already have.
you could very easily ruin your sound if you bought gear with the opposite audio priorities of what you're seeking eg. if you want more bass at higher volumes, acoustic suspension would be bad for you while if you want tighter and less distorted bass, switching to ported would be wrong.
if this is in regards to car stereo, yes, please move to the correct forum. if it's home audio you're talking about, what are you trying to improve on?
the only real reason to upgrade is because YOU are unhappy with your sound quality. if it isn't broke, don't fix it.
i don't own a car, so i don't think in car lingo. LOL
wrong forum, but anyways, car speakers aren't that great unless you have a mark levinson or B&O car stereo. "stock" speakers are cheap budget drivers that usually don't even have tweeters.
you very likely could improve your sound with just entry level gear but could do even better with component speakers.