it depends on what kind of sound are you wanting.. what type of music do you listen to???
if you listen to rap and want it to replicate those lows, then do what booze says and get a ported box.
Anything else, a sealed should do, as most music doesnt go below 60ish Hz unless its synthesized (like some rap songs).
With 800, i would get a new head unit to power your mids and highs (whether it be 2way speakers or a component setup with crossovers) but definitely do get new speakers, because stocks blow. Then get a box and sub and an amp to power that sub. If you install it yourself it would be a lot cheaper (need an amp power kit), but if you dont know how to do that then you need to take installation into account.