aftermarket speakers with mach 460

[ac97gt]

i went into best buy to look at some 4x8 speakers to use in my '97 GT with mach 460, and the kid there who helped me starting explaining how he was non-commissioned, and then goes on to tell me that if i just replace my speakers and not the head unit and everything that my system will sound absolutely terrible. now, i have no money, and the only reason im tryin to replace the speakers is because the front right speaker is blown. i just want something to last until i can afford to do it right. this could practically wouldnt even sell me the speakers he was so hung up on how its optimized from the factory and if you dont replace it all it will sound awful. anyone have any input? i was just gonna replace the 2 front 4x8s for now but im thinkin i might just do all 4 (front and rear) and then do the "tweeters" (2.5"? 3") later on. also the speakers they had were 4 way pioneers and 3 way kenwoods... the kenwoods were $10 cheaper but i thought they sounded better and the kid said they were a better speaker as well... any input? (eventually i will replace the amps and head unit and get some subs, but like i said i have no cash now for anything more than 4 new 4x8s)