what do you plan to imporove
id recommend adding sub to stock system, like this
http://www.mudah.my/Epsilon+8+active+subwo...ox-19622369.htm

it usually satisfies 95% of people, unless------

unless, you've been bitten by the bug, which no more common sense can cure/save you hehe

since you concern about boot space, id recommend portable player + earbuds

I don't chase sq anymore in car these days, I just use portable media player and earbuds if want to listen to serious musics, no road/wind noise, no more bass drowned by tyre roars when driving >60kmh, no rattling noise,,,,, just me and audionirvana two become one
if i want to listen to radio/traffic I can use the HU, which also has aux/line in, so I can plug the portable player into it when im driving with buddies/families,,,,,,,,,

some portable player can play lossless audio like flac, with 40GB it can store ~10 flac albums,,, syiok

really depends on how much you wanna go for your sq ICE. full entry level SQ setup ranges between 3-4k+.

that should include component set speakers, HU, 4channel amp (assuming you want to include sub, 2 channel if not), passive subwoofer in boot and some sound proofing on the door panels.

there are few installers that can customize the boot to fit the sub properly so you won't use up too much space, or you can go active underseat subs however don't expect much out of those