The keyword is bandwidth management and the Linux Kernel has direct support of it. I dealed with this long time ago and have forgotten much about that but there is a great online book that covers the topic extensively. It's called
To get a foretaste of what you want to achieve take a look at
Perhaps there are (software) servers which support bandwidth control directly!?
Supplement:
- In the pureftpd FAQ it is explained how to enable bandwidth limitation on the system level. On the first sight it seems to be applicable to other ftp servers as well. 
- thereThere is another freely available book called - that deals with the topic. At first glance it seems to worth a look.