Well....what do you plan to DO with said girlfriend?
Haha i'm kidding, but uh, let's list the pros and cons. Oh, and if you REALLY liked a girl already I doubt you'd be asking yourself this question.
Halo 3:
Fun!
Friends online
It asks for nothing back
Never goes away/unchanging
Cons: You have to sit there playing video games instead of getting out and experiencing stuff.
Not nearly as fun as some other stuff.
Girlfriend:
FUN! (more fun than Halo 3, if you catch my drift

)
Someone to be with
If you really like her, and she likes you back, a relationship can be an amazing thing
Impress your friends! (ha)
Cons: Time consuming, and not necessarily on your agenda
Requires a lot of love, which can be great unless you're not feeling it
It can be expensive! (sometimes)
Will cause you to compromise on some things
Well...ok I don't think the list does a good job. If it were me, I'd say stick to the Halo 3 unless you actually meet a girl you're interested. Don't just go out looking for girls like you're shopping at the black market. But as soon as you find one you like ditch the xbox and go for it! (maybe I'm just saying that cause I'm more of a CoD kinda guy)