rite ok if ur into dance and trance etc I would suggest a three speaker system (set of components to replace the exsisting speakers in the front doors and a single 10inch subwoofer) run off a 4 channel amp to keep it all tidy and compact and also to simplify wiring.
Headunit - Alpine CDE-9825RB £159.99 from Caraudiodirect.com (this headunit has the all important three pre-outs allowing you full control over ur system and also the usual high standards of alpine sound)
Amplifier - DLS CA40 £299.99 from caraudiodirect.com (this amplifier will be used to provide 2x50watts RMS to the front components and 1x170watts rms bridged to the subwoofer, doesnt sound like a huge amount of power but the amplifier gives a very clean sound and these are real world power figures)
Front components - Genesis profile 13 13cm front components £149.99 from caraudiodirect.com (havent actually listened to these but to be honest you cant go wrong with the genesis brand...I suspect these speakers wont be overally loud but will give a very smooth and relaxed sound which will be easy to listen to for long periods of time...if this isnt ur thing and u prefer a more powerful "in ur face" type sound Id probably suggest something from the infinity brand)
Subwoofer - Alpine SWS 1041 10inch subwoofer £89.99 from caraudiodirect.com (smooth sound once again with excellent punch and weight for a 10inch driver)
subwoofer enclosure - Single 10 sealed box - 0.75 cubic feet £39.99 from caraudiodirect.com (this size enclosure will give a nice quick punch but still allow a little bit of flexability outside of ur normal musical tastes)
amplifier wiring - Stinger SWKH8RS 8 guage wiring kit £67.99 from u guessed it, caraudiodirect.com
Additional 5metre RCA cable required to have full control over the level of ur subwoofer - Stinger SHR17 £9.99 caraudiodirect.com
total system price - £817.93