If you are going to connect a USB stick to the xbox, you need to mod a USB socket to the controller (that would be easiest). You can also mod the sockets to the x360, but that is not as easy imo.
Look at this site after you do that:
http://xboxsaves.com/MXK_HW_Guide.htm
You don't need a cross-cable to do a softmod. The savegame installs everything for you. However, using the save as a boot disk would be more stable than a dashboard exploit. I suggest you mess with that first as it is easier.