hehe okeee
basically a nice big fat amp, as the basic stereo in the zed will need one. I went for that alpine (google it) coz it's soooo small compared to others, and it's bascially overpowered for the size of the car.
but when you plug an amp in, you generally need a "pre-out".. which is like a signal from the stereo before the original amp gets hold of it, so you don't get any distortion before you new amp takes over.
the stereo doesn't have these, so you can buy a "cleansweep" which is an amazing bit of kit, listens to the sound out of the speakers and bends it so it's back to the original sound (of the CD, Mp3, whatever) then feeds the amp so you get a nice distortion free sound, but still keep your original head unit.
if you get the upgraded stereo on the Zed you'll get subs behind you, you can buy the bits from the dealer to put your own in, then buy some subs and connect them to the same amp.
Replace your front speakers for about £150 and you're set.
I actually find the standard stereo ok, it's just not loud enough when you're 70mph+ with the roof down.
Having the Ipod has made a huge difference too.