there's no need to change the discs if they are good, but get a set of EBC's. greenstuff for under 200 bhp and redstuff for over 200 bhp. they give of very little brake dust, use a compound that enables better braking in both the wet and dry without any fade. Brembo's are better, but there is a limit to the money you want to spend. i've had EBC's on several cars and they are great for hard progressive braking.
however, i wouldnt advise you bother changing until they need changing. BMW use a good quality brake pad in keeping with the type of car you have.
G