There's always interference between upload and download bandwidth: Say you download at full speed, then there will also be a little bit of upload going on because your computer sends out ACK packets to the server letting it know that the datagrams it sent actually do arrive. Now you start an upload while the download is still running at full speed. You will receive ACKs from the computer you are uploading to which require a little bit of your download bandwidth. Consequently your effectively available download bandwidth drops.
By how much down/up-stream bandwith drop when maxed out simultaneously depends and quite a lot of factors some of which you cannot control (only your ISP could tweak them). However, there's hope: Check out the Linux Advanced Routing and Traffic Control HowTo (
http://lartc.org). This is not exactly an entry level howto and if you don't care about the ins and outs of traffic shaping your best bet is to get the wondershaper (linked on lartc.org) script and let it do the work for you.
cheers, demian
