I would not replace old ones unless you replace the radio as well.You will not get the best sound that way and you can get a decent little radio for 100 dollars or so to go with whatever ones you choose, which system do you have the 4 speaker or 7 speaker with or without navigation system?
Basic speakers just front and rear jvc and pioneer are ok if you wish to step up for quality sound go with mb quart or infinity or jl audio.