Depends for how high quality sound you are looking for.
If you're looking for high end but not necessarily competition sound, you can prolly do really well with the infinity kappa components.
And for about $200 or less.
Get both the 6x9 and the 6.5" infinity kappa component speakers.
The components come with crossovers, opposed to just coaxial speakers.
This is what i have in my car, and these speakers are sex to my ears.
All my friends go to brag about how their subs are louder than mine, but when they hear how clear and clean my speakers are they shut up instantly.
2006 G6 GT
Mods so far: Black arrow overlays, Chrome recessed grilles, Blue LED license plate light, Blue LED reverse lights, Smoked tails, 20% Tint all around, Red led's for the interior, White led dome and map lights.
Sound System: 4 Infinity Kappa component speakers, 2 Amps (950w total) and 2 10" Kenwood KFC subwoofers, 7" touch screen Pioneer DVD
Performance: Mobile 1 oil, Mobile 1 oil filter