Definite improvement in sound just with a speaker upgrade. Any 6.5 inch diameter will go straight in.
Not sure if the amp is worthwhile to be upgraded - depends on what your wanting I guess.
My deck died it's second death so ripped it out and put a Sony head unit in with a front speaker UHF under it. Sony deck has a front USB port so plays away using that full of MP3s, no skipping, no jumping, and easily removed