Hey,
I just wanted to throw in here. I have an Alpine cd-9825 head unit in my 3 running two sets of Polk Audio 5.25 co-axial speakers in the front and rear doors. I left the factory tweets on the front pillars. In the back, I have a single Alpine 10" (E series I believe)in a .70ft3 ho-made box, hooked to an alpine monoblock amp @ about 200 or so watts to the sub. Granted, this is a cheapie system(less than $700), but it sounds pretty damn good IMO.(I'm not out to set off car alarms with it). In a hatch, you don't need to much bass to have it sound good, and 3's have a lot of rattles that pop up if you do. I'm in the process of ordering/installing sound deadening for the floor, trunk, and doors to finish it out. Also remember that you don't have to buy everything at once, you can do it in stages. It might help out with budget/making the right choice of components. The moral of my story is that more is not always better! This is all in an '04 3S Hatch. (alright)