Nowadays I think it is 8000+. All the N/A engines in today's top sports cars like GT3, M3, RS4/R8, Ferrari, Lamborghini all have 8000+ redline. 7000 to 8000 is decent for a N/A engine but is not cutting technology any more. Also I think you need a 8000+ rpm redline to break the magic 100+ HP/liter barrier (atleast I don't know any N/A engines that rev less than 8000 RPM that make 100+ HP/litre).