nothing will happen...

100mbps is the speed of the connection between your PC and your router.
So data will come in to your router at 512kbits (theoretical maximum of 64kilobytes per second)..and then transfers from your router to your pc at a speed of 100mbits (theoretical maximum of 12.5megabytes per second).

So upgrading to a 1mb connection won't make any difference.. your bottleneck (as with everyone), is the speed of the internet connection..

If you were at uni and had, say, a gigabit connection to the internet (like one of my friends in the states does).. then it would be worth upgrading your NIC to gigabit (and possibly going from Cat5e to cat6)..