One other, often over-looked item that I haven't seen mentioned yet: MTU. The mtu represents the largest single packet that can move from your network, through the ISP, to the destination.
Generally it's around 1500, and most routers default to this. HOWEVER, if you're using DSL or some other technology with a packet overhead (PPPoE), you may need to lower your MTU to keep from having packet fragmentation. Packet fragmentation has a DRAMATIC HORRIBLE EFFECT ON YOUR NETWORK SPEED.
As a simple example, if your PC has defaulted to 1500, but your DSL line can only take 1492 (because it's 802.3), every single packet you send will be fragmented into one 1492-byte packet, and 1 8-byte packet...You'll be sending twice as many packets, your collisions will be higher, your overhead will be higher, and your connection will suffer.