Well, the fastest speed for an ethernet cable is about the same as 802.11 B wireless- it will be slower than the speed you get wirelessly over 802.11 G. That would not be an accurate gauge anyway- that is only the speed between your PC and the router, not the actual net access speed from the modem.
Thing is, you should have plenty of speed through that cable to play without lagging on the broadband connection. Only other thing I can think of is the problem is in your internet access itself. Do you use cable or DSL? If it is cable access and a number of people around you use it too the problem may lie in the bandwidth there being shared among all the users. That's one of the disadvantages of cable over DSL, which doesn't get shared. If this is the case there may not be much you can do other than try to play at a different time when not as many in your area would be online.