jaslion :
Whilst you can get a usb 3.0 to gigabit ethernet you will not really notice the speedincrease. You will download things slightly faster but the problem is that you have a very slow 5400rpm hdd which will not allow you to have use all of the bandwith of your fiber since it simply can't keep up with it.
Sorry but that's BS Jaslion, 100Mbps is about 12MBps, so much much slower than a 5400rpm disk. 1Gbps, is around 100MBps in the real world, which should be achievable by a 5400rpm disk, so something that is at least 5-6x faster than 100Mbps.