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.
2006 GT, Strut Tower Brace, Gas Hood Struts, FR Intake Plenum cover, Metal Fuse Box cover, Moroso brake reservoir cover, Rear shock tower brace, Front Bumper Lip Spoiler, JVC DVD Head unit w/ back-up camera, satellite Radio and USB input, 2 - 12" sub-woofers, 4 - 6"x8" speakers, 2 - 6.5" speakers to replace the door subs, 3 amplifiers, LED lights under the hood and in the trunk, chrome e-handle on parking brake handle, chrome cup holder trim, and trunk lid cover with running pony emblem.