As others have stated in one form or another, your network speed
will be limited more by the source of the data than will be affected
by the cable being used.
A few examples:
-Internet speed will be limited by what your provider can deliver.
If you only have 25 Megabit or even 200 Megabit from your internet
provider, that is the speed you will have. Nothing in the world will
make it any faster.
-Intranet file transfer will be limited by how fast a connected machine will
push bits out of its ethernet port or how fast it can get data of the device
it has the data stored on. For a whole variety of reasons, it will never
achieve a gigabit data throughput rate.
Bottom line, don't worry about something that is not an issue. If sometime in the
future when computers and internet service providers put 10 gigabit ports on equipment
as a standard, then worry about cables.