id start off by doing a lil resurch and finding a good head unit for your car
www.crutchfield.com could probly point you in the right direction (very expensive place to buy from though) for most things
looks like the dash speaker is a 3 and a half inch round and the top-mount depth: 1-9/16"
rear deck suposidly fits 6 and half inch round speakres with a mounting depth of 1 7/8
with just a factory head unit and new speakers man it will improve the sound drastically and will be better sound then 95% of anyones vehicle with a factory setup
best way to tell what the best speakers are for your budget is probly go to best buy and listen to them all......