Probably not so easy with a Windows machine these days as our friends at Microsoft started locking the Windows networking components down in XP to prevent promiscuous captures and I expect the case is the same with Windows Vista/7/2K8. I have an old laptop running Redhat and an ancient distro of Ethereal which works just fine with it's internal NIC.
Just make sure your hub is actually a hub and not a switch if you want to go down this route
Just a quick footnote: This is where your dd-wrt comes in handy, you'll have more diagnostic tools in the little box than you can shake a stick at. I used to have a cracking d-link router (non dd-wrt) which had a connection status dialog for the wan port in the gui, I expect this is what you had before. Alas, useful functions like this seem to be disappearing unfortunately.