cable internet very slow

Very very slow comcast internet?
I have comcast internet and it is really slow. I live outside of Boston. I have a pretty new, nice router the d-link dgl 4500 and a toshiba PCX1100U modem. I have an ethernet cable attaching the modem to the routers wan port.
I have no idea why this is so slow. I know that the router should work well and that comcast is pretty reliable. Could it be my modem
I am always lagging out of games and the internet is really slow. If i do a video chat the internet is unusable for anyone else. Please Help!
if i’m in call of duty it will say 4 or 3 bars but i will still lag some and then halo 3 it will just be red connection (i don’t know if this has to do with my connection to the servers though)
go in to your comand prompt and type the following
ipconfig /flushdns
this should help you
Dial up – worst internet connection ever!
