That pcb/kit might be perfect for it. The shiftbars would work well too.
The ledpainter uses a 15 dollar board, and 3 5 dollar texas instrument IC's...among a pile of other components, and might not support much current on each channel (no led bars probably)
The shiftbar is about 9 bucks a piece in small quantities, you would need 1 per led bar 'channel', somewhere between 2 and 8.
For a realistic amblight you only really need 2 channels for the effect (left and right), and more just amplifies the effect. '4' bars would be perfect, 2 sides and 2 on top, which would mean you just use the built in arduino mega pwm.
You hook up a 12 volt source to transistors (mosfets if you start drawing more power), and use 5v from the pwm lines of the arduino to switch the transistors (or mosfets) on and off.