I have Cox cable internet, and when I'm connected just to the cable modem, the speed is great. But when I connect my Linksys WRT54GS v.3 router up, the speed is worse then dial-up. My cable modem is a D=Link DCM-200.
The MTU for Ethernet should be around 1500 I think. Unless you've changed it, I think that's what the default is. I would try resetting the router to defaults, if that doesn't work, chances are something is wrong with the router. If you had another router you could try, that would tell you for sure if it's the router itself.