I have an old 2007 HP slimline - s7700 - when I download something on this system I get around 350 KB/s with about 100 - 110 (or so) KB/s upload - it seems to have upwards of a 100mbps card (or whatever).  I also have an Acer - Aspire one (bought in 2010)  on it  it can get upwards of a Mega a second. It seems to have a gigabit ethernet card (or what ever). With what we supposedly pay our ISP it'd seem like I should be getting ~350 KB/s or probably less than a mega. I think both computers were made around the same time (one in 2007, the other in 08) - they should it seem to get around the same speed. Why the difference?
 
I am going to sometime this year get a new computer - I'd presume I'd still probably get about the same speeds as I currently do. When I do a test on speedtest I get ~ 3mbps download so regardless of the card (100 mbps or a gigabit) it seems silly to have that much when I'm probably never going to get that much. Our router is ISP brand - It (and the computers) can only support wifi B/G - no N.  I am in Texas BTW ..  .

I think you may have the concept of megabits (Mbps) and megabytes (MBps) somewhat mismatched. In terms of data a bit (b) is a single binary digit of 0 or 1 and a byte (B) is 8 bits.
 
In terms of your ISP connection of 3 Mbps you should have a download rate of 384 kilobytes per second (KBps) which is what your browser registers when you download a file from the internet. From your download rate mentioned in your post, I think this is on track.
 
Using a simple online conversion site you can convert 3 megabits to kilobytes and it comes up with 384 KBps.
 
On your internal network transferring from computer to computer you should get much faster speeds. You said that you have a b/g wireless router which has an output of about 54Mbps at 100% signal strength. With a perfect signal it should equate to ~6 MBps over the wireless which will degrade with the signal strength.
 
Simply put, it sounds like you have the correct amount of bandwidth from your ISP.
 
Hope this helps.

aside from a LAN unless its 1k across the board you will not notice the difference between 100 and 1k.
 
and yeah above poster basically put it in place - im guessing your NIC is a decent one so you really shoudln't see much of a different....
also don't forget most personal connections don't get a guaranteed speed whereas businesses usually pay a large premium for it.

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.