I have been struggling with this problem for a couple of months and I've yet to come up with a solution. I currently have high speed cable internet running to a router (ASUS's WL520gu to be exact). It's a wireless router, but I'm connecting my two PCs wired, and using the wireless very seldomly for the Wii which is a foot away from the router.
When I first upgraded my modem and internet package, I was getting download speeds of upwards of 1mb/s, but it seems that ever since I got my new router, I'm really lucky if I get over 50kb/s.
I have my Xbox 360 hooked up to my router wired, too, and I was downloading a movie from its marketplace and downloaded 10% of 1.4gb in an hour. As an experiment, I plugged it directly into the router and the remaining 90% finished in ten minutes.
I figured someone here might have a bit of network experience (looking at you, digi ;]) and wondered if anybody knew what was the problem. I've tried googling it but can't seem to find an answer.
tl;dr - my internet is very fast when plugged directly into the modem, but very slow when plugged wired into my wireless router. what gives?