It really depends on what you want to acheive. (best sound quality or volume)
if your going for a high 'Sound Quality' system, then active crossovers and multi channel amps (or ideally seperate amps for each set of speakers, F/R) plus either a dedicated channel (as in a 5 channel amp) or a dedicated amp for the Sub is a good set up.
However the quality of the sound will only be as good as the weakest link in the system, so now you need a HQ head unit (or digital source as Meddler has) and a clean signal to the boot where the amps/xovers will be kept (if that's were you intend them), options here could be a 'Line Driver' i.e. high voltage signal so that the signal to noise ratio is low. Or a digital connection like optical or digital. Check and remake your ground connections, far too many installations suffer because of interference/noise from poor ground connections.
Then some form of equaliser (1/3 octive, parametric, digital etc) to correct the accoustic errors (peak/troughs) within the car and then some set up time with an RTA to 'tune' the sound.
Depending on how much you have spent on the equipment and set up this is a simple competition set up. (minus sound deadening and many other work to be honest).
For a street system
I'd say 5 channel amp (2ch front, 2ch rear, 1ch SUB), some decent components in the front (say 6.5" 2 way ideally with adjustable tweeter positioning) with passive xover (but using the amps built in xover as well to limit the low end to the front stage).
Small 4-5" rear effect speakers on the shelf (fairly centre, no need to seperate out and 2 way, not dual cone) again passive xover.
I really don't like rear set up's, it's completely un-natural and ruins the front sound stage when it's over done, it's meant to simply provide some rear effects simulating the boundry reflections from the front sound, not a new sound direction in it's own right, after all when you listen to a band there not playing from behind you...
Then a good quality Sub, 10" should be all you need, spend as much on this as you can as the sound from poor subs is completely different to high end subs, quality would be JL Audio W6/W7 standard. Then depending on what sound you like, sealed or vented box. Feed with the remaining channel from the 5 channel amp.
That or if you can afford it go fully active on the xover and bin the component xovers.
Keep it simple, spend some money on angeling the front components and the sound quality can be amazing. Best to get help once you have the gear from a good car audio installer on the final speaker positioning and setup, the difference can be night and day.
Really there's so many way to do it these days on the front end (PC based front ends, laptops, headunits etc), but on the rear end (processing, amps, speakers) it hasn't changed for years (different stuff but nothing revolutionary)
If you want loud then that's a different story
Many years ago now I spent two years competing in the UK SCA (Sound Challenge Association) circuit in SQ+ (Sound Quality + SPL), made it to the finals at Earls Court as well
