Keep us informed! I would hope you could get $35-$37K at Lexus in trade and $38-$40K selling on your own. A dealer other than Lexus I would expect probably $31-33K :( . I also have an '08 400h, (but 2wd) and premium pkg and love the car, but not the payment (starting to think it is more car than I need), so I am curious as to what you get for yours.
By the way, my local dealer has this GS 450h Hybrid that may or may not interest you:
http://www.eskridgelexus.com/VehicleDetails/329722663