100Mbit or 1000Mbit NIC don't mean much for internet connection if you cable modem is DOCSIS 2.0, because you are limited by your cable modem speeds and ISP networking speeds and modem's NIC speed.
it gets much better with use of DOCSIS3.0 cable modems .
100mbit and gbit nic will make hell of a difference in your LAN, but in fact your cable modem connection can be max 100mbit, thus making the physical link to your computer (even if going through a gbit router) still at 100mbit.
also as prev poster said bits are not bytes, you need to divide number of bits by 8 to have the speed in bytes, then K & M factors are done by multiplying by 1024 (not 1000 like in physics).
and so the 100Mbps speed equals 12.5MB/sec at maximum specs (not a real world value) also depending if connection is Full Duplex (same speed both ways) or half-duplex, etc those things are auto-negotiated by devices connected over cable.
gbit NIC will be limited by design at 125MB/sec at maximum (you wil never see it in real life application).
now, once you connect all devices in your LAN network together and some wifi router and even more devices over wifi, it will create some traffic. if it is congested (e.g. everybody actively using networking connections by transferring files, listening to music from internet, watching videos, MMO gaming, etc.) you can expect around 30% of the max speed... and so gbit NIC real speeds will be around 40MB/sec and 100mbps will actually becomes something like 4MB/sec max <- this value might be what you can get max from your cable modem if it carries only a 100Mbps NIC port (e.g. a DOCSIS 2.0 device), otherwise if you have a modern DOCSIS 3.0 modem then you can expect around 50-70Mbps speeds at max depending on your ISP network congestion, number of clients and a hundred of other things.
also remember that you connection speed is limited to the slowest hop on the route between your computer and the ISP, so if you have an old router supporting only the legacy 100mbps NIC speeds or B/G band WiFi then it will not ever get faster than those specs.
If you have any kind of DSL connection to internet, then I can only say I'm sorry, but it's an ancient technology.
hope this helps to understand why you are getting speeds like you do.
you can use www.speedtest.net
to test your ISP connection speeds at different times of day to compare results.
<message edited by feniks on Wednesday, April 18, 2012 9:15 AM>