Hello there! If you can, I would definitely give the ring during the proposal. Is there a reason you were thinking of doing otherwise? Did your gf express a wish to pick out the ring with you, or any other reasons? I guess it depends on the situation, but I know I would want a ring just because it seems more official that way for me - esp. because my boyfriend and I have talked about being engaged and whatnot, him asking me without a ring would seem like just another one of our conversations!
But it really depends on what you are comfortable with and whatever you think will make your gf happy. Good luck.
Most definitely present the ring with the proposal!! Even if your GF helped pick it out, I think it should still be presented with the proposal. Otherwise as ClownFishFunk pointed out, it is like having a conversation about getting married!