What's wrong with asking her out on a date?
Call her tomorrow or wednesday and ask her out on a date for friday or saturday night. If you wait much longer you won't be very fresh in her mind. That, and if you wait too long she'll think you're some sort of chump who doesn't have the balls to call. Or you're "just not that into her."
Don't be afraid of the word "date." You meet a girl and get her number, a date is the next step. If she's just hanging around you one night and you haven't made it clear that you're on the first date she might be kind of confused about what's going on, whereas if you go ahead and throw the word "date" out there, she understands what's going on. You don't necessarily HAVE to call it a date, but it doesn't hurt. It makes your intentions clear. (this isn't a hard and fast rule, it's just something I find to be a good idea)
Anyhow, the first time you go out alone with a girl you haven't been LJFBed by, she pretty much assumes it's a date. Roll with it.