The Luxury I package ($3,050) adds park assist, heated and cooled front seats, heated rear outboard seats, rear seat adjustable headrests, four-way power rear lumbar support, heated windshield washer fluid, Rainsense Generation II intermittent wipers with structureless wiper blades, heated power-adjustable mirrors with turn-signal indicators and driver's side auto-dimming, leather wrapped and heated steering wheel, and tilt steering column.
Luxury II ($2,340) includes all the content of Luxury I, plus tri-zone automatic climate control and a universal remote transmitter.
Luxury III ($5,075) includes Luxury I and II, plus an upgraded stereo with a six-disc in-dash CD changer and eight Bose speakers, cargo net, IntelliBeam headlights, front four-way massaging power lumbar, power tilt/telescoping steering column with memory settings, trunk mat, rear illuminated vanity mirrors, genuine Dark Burled Walnut wood trim and 17-inch chromed aluminum wheels
The Performance package ($7,560) includes all but a few of the Luxury I, II, and III items, plus the higher-tuned 292-hp V8, performance algorithm shifting, Magnetic Ride Control, and P245/50 H-rated tires on 18-inch chromed aluminum wheels.
The Platinum Edition package ($6,490) combines the 292-hp V8, Performance Algorithm shifting, Magnetic Ride Control and 18-inch chromed wheels with special materials on the inside and an extra dash of flash on the outside; a power sunroof, DVD navigation, and other conveniences are included as well.
I don't believe there was a 'premium' in 2008 -- I think they added that in 2009