sometimes, when one blurt out that there is love, it might even
spoil the whole friendship.
Its platonic relationship between guy and gal..
I think best friend can also come from opposite sex. But try not to
be too close, because being veri good frens and love is a thin
line. Unless yr gal friend are those buddy buddy types
lah....
If not, both parties are attached...then being close frens might
not be a prob(unless your own partner minds) 
BTW, regarding asking her out on valentine day, i think you might
have fall for her, even if its a tiny winy bit of love. Actually,
you never know if she also have likings for you too right?
Some pp takes years before they come together...
Jus ask if she has plans for that day, and if she doesnt, ask if
she wants to go out dinner with you lor..see how she reacts...if
she says NO or something awakward, try to lighten the mood by
saying "joking lah" or " i tot you might be too bored"...
If you still have reserve for it, i suggest you dun ask her.
Instead, just send a a bouquet of flower on that day...