I replaced mine with some pioneer 3-way speakers in the 6x8 size. They are rated at 40 watts per channnel RMS continuous power and 350 RMS Max. I purchased a Hifionics 4 channel amp that runs 4 channels at 40 watts RMS. That's more than enough power for me. However, if you are into competition or need to have bragging rights among your peers, you are free to go with higher wattage in speakers and amps. I suggest that you listen to the lower wattage and the higher wattage at a car audio shop, before you spend lots of money on power that you wont use for everyday listening.
I have 2 - 8" polk audio subs in the trunk and They are also adequate. I like my music relatively loud and these do it without any distortion.
I attached a photo of my completed install. I did it myself after considering a local audio shop and Best Buy. Glad I did it myself. Good luck.
Nice, however, as of March 30th I no longer own the V6. I traded it in and got a '03 GT with a manual tranny. However, before I got rid of it I installed a Sony XAV-622 head unit along with a Rockford Fosgate P3D412 12" subwoofer powered by a Rockford Fosgate P400-1 pushing 538watts @ 2 ohms. I did strip the 6 banger's audio out since I only had it in for a month and put it in my new GT. The GT has the Mach 460 system and I love the stock speakers/amp combo so I'm going to stick with them. I may add another another P3D412 powered by a Rockford 1200W Prime series amp.