are stock ones maybe get 5w RMS, please dont get me started on my frustration with people quoting max power (or IfLightingStrikes) power ratings.
any aftermarket speaker will be better than stock, keep it 4 ohm if you have a stock headunit. Amped speakers is a large upgrade, but if you're worried about a 300w rms sub amp, why aren't you worried about a 250w speaker amp?
I would stick with the sub, they add a lot to music. Speakers can't hit that low and not only do you take the load off of them by not asking them to play so low, you can now hear those low notes in clarity.
my personal preference is round speakers, the 6x8 hole can be adapted to fit a 6 1/2, the rear takes a 5 1/4
any more questions feel free to change to PM, as I don't think we need to keep resurrecting this thread