there's not really a line... more like a nice fuzzy grey area.
if you use something a lot, like a graphics driver, then it's probably best to include it in the kernel. if it's something you use sparingly, just once or twice a day, like a sound driver or pppd, then it might be better to make it a module. the smaller the kernel is, the faster and more stable it is. but the difference might not really be noticeable
. it's just a judgement call, and it's not really worth losing too much sleep over.