You should go into Halls for your first year.
From my own experience, when I first went back in 2000, you are put into a situation where you are in a flat/corridor/building with a load of people in the same boat. It is slightly weird and awkward at first, but you soon get to know each other very quickly. Fresher's week still sticks out as a brilliant time and I don't even mean just the going and drinking, even some of the stuff we did in Halls was good fun to.
The people you meet in Halls will most likely be friends for the rest of the time you are at Uni and you may even move in somewhere with them the next year, in my case it certainly was.
Now, you are sure to make friends with people on your course also, but from what I saw, the people who came into Uni didn't have quite the same experience. I remember one guy I worked with asking me about it once and he wished he had gone into Halls, rather than staying with his parents.
Another thing I might add, is that in my fourth and final year at Uni, I went back into Halls, as most of my friends were on 3 year courses and had left. That was a totally different experience to my first year, it was worlds apart. Everyone is final year and so very busy with coursework and exams and everyone also has their own set groups of friends already. So nobody wants or really needs to get to know anybody else that they are living with. I chatted to some of them in the kitchen when cooking, but other than that, I didn't know them. Infact, I even had a guy living opposite me that I didn't even know was there!
The final factor, as unpleasant as it may sound, is that most relationships that are going on before Uni end when people go to Uni. I saw a lot of relationships end in the first couple of months, some did string it out for about a year, but even the ones that did all eventually ended them. So as harsh as it sounds, don't go out of your way to live with your girlfriend, you will have a much better time in Halls.