Theres a ton of aftermarket radios, and some reviews on here. Just search around. But if you're gonna get a tune (programed), and you plan on doing the intake and exhaust, get the intake and exhaust first, because the tune needs to be programed for it. The best intake is Crabs. (crabintakes.com), the best tune is WAMS (just google pontiac g6 WAMS), and as for the exhaust, that is all a matter of opinion.
If any question come up, try searching first, and if you can't find it, post it. There is a large amount of knowledge on this site, and everyone is here to help each other.
CRABS o3, Custom Magnaflow Exhaust, WAMS Performance tune, Racingline STB
Platinum Apex Rims (18x8), Hankook v2 215/45R18, Painted calipers (Red), Complete debadge , Handle Overlays (Gloss Black), Arrow Overlays (Gloss Black), Grille Lighting (White LED's), Tinted Tails (2 coats VHT Niteshades), Painted Battery Cover (Silver with Red), Painted Engine Cover (Silver with Red), Eagle Eyes Projector Headlights (Black Housing), Upper Recessed Grilles (Chrome)
2 12" Memphis Subs (15-MCP12S4)
600watt Sony X-plod Amp