please find it here for the difference. http://www.kenrockwell.com/nikon/sb600vs800.htm
for Nikon D60, SB-600 will be a good combo. personally, i like SB-800 over SB-600 only for its built-in bounce card.
would not like to recommend SB-400. you may end up with selling it out and change to SB-600 or SB-800 soon
:cheers:to your better flash photos
no to SB-400 unless you are not serious about lighting go for at least SB-600 and above.
Do note that when you buy equipment for DSLR, especially true for flash, it is sunken cost
as the item will wear out.