I have a 2000 GT and I upgraded to Infinity Kappa's and they KICK-ASS. Even with the stock head unit. I have no amps and they still crank about 10 times louder than the stock speakers. Got them on Amazon and paid dirt cheap for both front and rear speakers. The fronts have seperate tweeters so they mount just like the MACH sound system right above your side mirror adjustment knobs.
Pioneer, I lost the Mack 1 system in my 97 and had the whole pioneer system/head/4 speaker put in, thing sounds superb. Louder than the Mach system, and only cost 500 total with install. from best buy. Plus the discount for buying all pioneer/promo knocked it down to 390.