Yes, it will speed up your connection between your PC and XBox. The difference between half and full duplex is loosely like comparing walkie-talkies and a telephone. With walkie-talkies, only one person can talk at a time, unlike a telephone where both people can talk at once. Basically, with full duplex each system can send and receive at the same time. You can only use Full Duplex when you have two NICs and a crossover, or a PC connecting to a switch (Not a hub).
This will only help if you are using TCP to talk between systems. If the connection between the two is UDP, it really won't matter as there is no need for two-way communication.
Assuming that the Xbox and PC HAVE a 100 Mbit capable nic in it, you can set it to 100 Full Duplex with a crossover cable.
Now with your broadband connection, it will help in the TCP traffic between your modem and your PC by preventing packet collisions, but I wouldn't expect drastic improvements in downloads. Pageoliver pretty much has it right. Most modems support a 10 Mbit connection only because the broadband connection is well below that speed. A T1 line is 1.544 Mbit and Cable Modems peak under 4, so even with the SYN/ACK packet overhead involved with TCP traffic, it can still easily pull that off within the 10 Mbit cap.