you could defiantely do better
for subs i would definately give reosnant engineering a call
www.reaudio.com because you can get he sx directly from them for an awesome price. For the amps i would look for these
jbl bp1200.1
american bass cx1100d
dei 1100d
tsunami 1100
those are all reasonably priced 1100wrms producing amps and can be scored on ebay for dirty cheap.
then i would look for another amp like jl 4 channel to power the mids and highs and you can snatch tha off ebay for cheap too
for components i would recoment the cryastal components there cheap on ebay but get the silk tweeters they sound better then the ti or alum ones. that will take care of your front and in the rear i would just upgrade to a higher end 6x9 or 5x7 or what ever you have and wire them all to the 4ch amp
for a headunit i would look at the eclipse HU or if you want ultimate SQ nakamichi cd-45z.
the subs will be like 400ish total or less if you decide to go with 1, the HU will be 300ish, speakers will be 150ish, and the ampswill be 350-400ish
similar cost but alot better equip.