Don't get her anything and judge the reaction.
If she gets pissy and angry, dump her. She is going to make your life a living hell. She is going to try to control you and make you do everything she doesn't want to do, which will be everything. "Honey I'm going out with the guys tonight" "No you aren't American Idol is on and you're supposed to watch it with me".
If she crys or is rather sad or upset, dump her. She is not emotionally strong enough to deal with life and again will probably end up making your life a living hell. This is the kind of girl that goes psycho after a certain period of time with thoughts of babies and wedding dresses. Get out now. While you still can, before you wake up and she's carved her name in your chest with a dull butter knife.
If she has no reaction and is like 'meh whatever', dump her. She doesn't care about you and is probably getting her V-day gift from some other guy. Which means your v-day gift might be some vaginal disease she picked up from his skuzzy ***.
If she got you something good and doesn't care what you got her because she is excited to see if you like what she got you. Keep her. Drain her for everything she has. Then when you're 40 dump her for the 18 year old Puerto Rican girl who is taking care of her sick mother.