Ignore huntluck
There are couple of things to understand ...
10/100 is measured as Mbps (megabit per second)
this is VERY different from MBps (megaByte per second)
A bit is a single piece of data (a 1 or a 0) a byte is 8 bits
If the "b" is lower case it refers to bit, if upper case it refers to Byte
10/100 Ethernet will run at 10Mbps or 100Mbps depending on each piece of hardware that is connected.
Gigabit Ethernet is 10/100/1000.
For example, a DSL modem that was built in 2001 will have a 10Mbps port on it. If you want to share that DSL internet connection, you would hook up a router like a D-Link DIR655. The gigabit WAN port on the router (which is connected to your DSL modem) will operate at 10Mbps to match the modem. You connect a old desktop from 2005 which is using a 10/100 ethernet adapter to the router. That port on the router will operate at 100Mbps to match the card. You connect 2 new desktops with gigabit ethernet adapters. Those ports on the router will operate at 1000Mbps.
SO ...
Any data that is coming from the Internet (through your modem) will be capped at 10Mbps.
Any data that is transferred between the old desktop and one of the new desktops will be capped at 100Mbps.
Any data that is transferred between the 2 new computers would go at 1000Mbps.
While typing this reply I moved a 1.86GB file between my 2 computers at 100Mbps and then at 1000Mbps.
At 100Mbps the file was transferred at 11.5MBps and took 165 seconds (2 minutes 45 seconds) to copy.
At 1000Mbps the file was transferred at 55MBps and took 35 seconds to copy.
Real-world transfer rates depend very much on a number of factors. Some of the bandwidth is reserved for overhead. Normal hard drives cannot read and write information fast enough to fully utilize a gigabit connection. I think WD Raptor drives are close, but for that kind of money you are better off going with a SSD (solid state drive).
As you can see from my results, I was able to saturate the 100Mbps connection (100Mbps/8=12.5MBps) however, I didn't even come close to saturating the gigabit connection (1000MBps/8=125MBps). This is because of my hard drives. I am using 1TB Seagate 7200.12 in one computer and 500GB Seagate 7200.11 in the other.
That being said, moving the file in 35 seconds instead of 165 seconds makes the choice between 10/100 and gigabit a no-brainer really.
Sorry if this post is a lot wordy ... I wanted to be clear, if not concise