I think they are also ways to split the load.
some applications can use one network adapter, and others another one.
more difficult to setup, but the end result is eventually a PC with twice the bandwidth.
using max capacity in both connections.
HOWEVER, you have to remember that most trading software, data software etc use very little bandwidth compared to broadband. so this might be a futile idea.
and again, it's possible to setup internet browsing on one network card and other activities with the other one. so downloads, browsing don't take anything to trading.
still, few advantages to all this. use DSL if you can. if you can have a leased line, well, end of discussion. believe it or not, for most traders a 56K modem is enough (to trade).