Apple yesterday released a new freeware application that allows you to take full advantage of your broadband connection, its generally for connections with high latency (5Mbps or greater) but will work with those that are slower.
The applications simply adjusts the size of the send and recieve buffers allowing more data to be transmitted in one go.
The following system parameters are set as the following, they are sysctl variables;
net.inet.tcp.sendspace: 131072
net.inet.tcp.recvspace: 358400
kern.ipc.maxsockbuf: 512000