hello,
first of all, since you're saying your NIC si gigabit lan enabled means that should you ever need it or be in a network where everythign communicates with 1000baseT cables your NIC can work effectively. Gigabit is needed when you're transfering large files from computer to computer via network and in home environment a gigabit ethernet is a waste, as the 100Mpbs is more than enough.
Gigabit, at this time and in my opinion, really only needs to be used in a corporate environment with heavy network traffic. At home, it's hard to find home users who truly need those kinds of speeds to transfer data between computers.
ISP connections are not as fast as your home network, thus the 100mbps connection. ISPs bottleneck the connection speeds to 100. I'm not aware of any home connections that really exceed 100Mbps. Even then, realizing those speeds is very hard. Using a 100Mbps adapter you don't really get the whole 100% performance, however having a gigabit NIC will offer a 100% effectiveness on the 100Mbps adapter (NIC).
Gigabit gets a lot of hype for it's speeds but it doesn't help out your internet connection speeds. That solely falls upon your ISP. Gigabit, in my opinion, really isn't needed in the home user market.
Stick with 100mbps and you'll see the same speeds as if you switched to 1000mbps unless transferring huge data files between computers. You could go to device manager and open up the properties of your network card. Set under the speed 1000Full duplex as the settings. Even restart if you wish and then go to view the status of your internet connection (n the connections folder). You will see that nomatter the specific speed which you have now set at 1000 the connection is still 100.
SO..you connection was never at 1000 unless you worked for EA games, Ubisoft, Microsoft etc etc and companies like that where transfering >1GB files over network is necessary.
another gigabit limitation:
The problem is how the Gigabit Ethernet chip is connected to the system. If it is connected to the standard PCI bus, it probably won’t achieve its full speed. PCI bus works with a maximum transfer rate of 133 MB/s, while Gigabit Ethernet runs up to 125 MB/s (1.000 Mbps / 8 = 125 MB/s). By just observing these two numbers you could say that Gigabit Ethernet “fits” PCI bus, but the problem is that PCI bus is shared with several other components of your system, thus lowering the available bandwidth. So, even though in theory Gigabit Ethernet can run fine on PCI bus, it is just to close to the bandwidth limit of the bus.
PCI Express, on the other hand, has a maximum transfer rate of up to 250 MB/s and is a point-to-point connection, which means that it doesn’t share this 250 MB/s bandwidth with any other device, thus allowing Gigabit Ethernet to achieve its full speed.
How can one tell which bus Gigabit Ethernet chip is connected to? There are three basic ways. The easiest way is to see if your motherboard is based on PCI Express bus. If it doesn’t, the Gigabit Ethernet chip can only be connected to the standard PCI bus where will perform as if it was a fast ethernet (100Mbps).