Rob.
With the increased level of road/exhaust noise that you will suffer in an estate.

your best option would be to install an amp and uprate your speakers.
Also change your original fit speaker wire to Oxygen Free Copper (OFC) wire as this will help.
4x40 watt is probably peak per channel and could be as low as 15-20 Watts RMS..... Not very much.
Unfortunately this is one of those subjects which gets more expensive the better you want it, depends entirely on how far you want to go.
Speak to an expert. If you don't have one locally try Car Electronics in Nottingham on 0115 924 0666 Justin or (Chris?) even if you only use them for ideas/options Etc.
They should be able to give you an idea on kit and cost to achieve your aim. any problems E me direct.
Mike.