Be sure you are certain you both are at the same place in life. Not trying to be stereotypical here but many women(at least around here) by the time they hit their mid to late 20s are ratcheting things down and are looking to "settle down". Many are and many are not but it would be best before you got too serious with this woman that you know where you both stand. But good luck...nut it up and ask. Worst thing she can do is say no and trust me there's a whole lot of things a worse than being told no. Like yes for instance.
just kiddin...