They probably arnt any more than 25w per speaker. You would be better off relacing the headunit, amp(s), and speakers.
I already have an aftermarket stereo deck (Pioneer AVIC Touchscreen Navigation, DVD). I remember watching a youtube video of replacing the whole stereo system and the factory amplifier was mounted by the driver side door making it extremely hard to wire things through to get your speaker wire in there. I went down to a local audio shop and I got a quote for 7,000 dollars. It better be well worth it, although that is all top-notch stuff. Although it has to be more than that, by far. That is what you would expect from a stock stereo deck with no amplifier. With my Pioneer deck and the factory amplifier my speakers begin to clip at 18/40 volume which is already very loud!