There's not much that will fit as a direct swap for the standard fronts as they are an odd size at 6 inch and have an odd mounting pattern.
If you're still using the standard head unit buy these adaptors
http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_188910_langId_-1_categoryId_265888
And then look for some 6.5 inch coaxial speakers to fill the hole. Personally i'd look past Pioneer and look at something like Focal, Hertz, Ground Zero, JL Audio. All can be had for about the same about as Pioneer speakers but will be much better. If you're going to add just coaxial speakers make sure you disconnect the factory dash mounted tweeters as well.
Anything on this site will be decent:
http://www.caraudiodirect.co.uk/speakers/full-range/6-5-inch-coax-17cm
If you have an aftermarket head unit i would consider adding a small amp to the system and getting a half decent set of component speakers. Something like this JL Audio amp would be good
http://www.caraudiodirect.co.uk/amplifiers/two-channel-amps/jl-audio-xd200-2
Really tiny and light amps but have a huge punch to them, and they can really be mounted anywhere in the car, Then i'd look at a decent set of speakers again anything by Focal, JBL, Hertz, JL Audio, etc. Just make sure the speakers you buy don't have massive magnets because the opening in the door isn't massive and you'll have to cut the door skin to make it fit and thats a pain in the bum
http://www.caraudiodirect.co.uk/speakers/component-systems/6-5-inch-comp-17cm
It all depends on your budget. If you've got less than £150 to spend i'd recommend getting the speaker adaptors and some half decent coax speakers and then disconnecting the factory dash tweeters, but if you want the best sound I'd get the adapters, the small amp and then some decent component speakers. It's not a massive job but if you're not handy with spanners get your local car audio place (not Halfords, a proper car audio place) to get you a quote, shouldn't really be more than a few hours work for them
Jack