Well, i have good new and bad news.
The good news is that your router is quite sophisticated and it has a section called "Bandwidth Management".
Also good is that you can give different types of traffic, different priorities.
Types being WWW, FTP, VOIP, Email and Video.
So at least you can give Voip high priority, the rest of the traffic will automatically get less bandwidth.
It's wise to shut down the Voip program when not needed to prevent it from hogging your bandwidth when not needed.
There is also subnet priority.
This is where the bad news starts. You can not give a single IP priority in this modem, only a subnet.
That would mean you would have to put your computer in a different subnet as your girlfriend's.
Example. Your computer 192.168.1
.20 and girfriend's computer 192.168.2
It isn't a real problem, but it could take some extra effort to make a network between your comps because they are on different subnets.
Here you can download your router's manual:
You will have to do the reading yourself.
Read up on the "advanced - bandwidth management" section first,
and then the "wizard, bandwidth management wizard" section.
See the screenshot.