A. 100Mb isn't gigabit. Giga=1000.
B. I highly doabt you have a 100Mb connection.Unless your paying 45,000 dollars for a OC3 line, or the test your taking is inaccurate. Or your looking at the connection rate, not the throughput.
C. If your Xbox caps at 10/100, you can't get above 100Mb network speed. The Chain is only as strong as it's weakest link.
D. You won't get 1000Mb from a adapter. Theoretical limit is 432Mb, with a real life usage of about 80Mb. So going USB is only only going to slow you down.
Long story short, unless your using your Xbox as a Multimedia unit (which is possible) then don't worry about it.