The moral of the story with any ICE system as far as I am concerned is - quality, not quantity.
As Samuel is trying to point out, if say, for example, your budget was £300 for adding an improvement to your system, dont waste it on 3x pairs of halfords speakers!
Instead, I would budget, say £150 for a decent set of front component speakers (and disconnect the rear doors and rear shelf speakers).
£100 to buy an entry level 2-channel amplifier (again, emphasise on quality and not a max power 800W driven into 1 ohm amplifier with flashing blue lights and all the bulls**t!)
And the remaining £50 I would spend on some decent cables. If you feel like one more worthwhile addition, I would suggest some some deadening material (for the front doors). It really isn't that expensive, makes a marked improvement in both sound quality and reduces cabin noise somewhat. And after all, it would be the best opportunity to install it whilst the door cards are off and the speakers are out!
Trust us on this, you really won't be disappointed, and stop having a mental block about disconnecting the rears, this WILL provide a much, much better soundstage and will give a far superior sound to any 6x9's your mate has in corsa!
Hope this helps,
Will
P.S, no offence Samuel, but why plough £8000 into a peugeot 306?
Surely even if you had to reduce your audio budget slightly you would be better off buying a Mercedes and spending less on audio kit?You can always add to it later on. I would still rather have a even a 190E than a 306 any day!