Eat at any of the fine restaurants listed.... and I agree Grizzly House is fun but not so romantic, if you want the fondue thing in a romantic setting both Banff Springs and Lake Louise have unreal fondue places (get ready to spend $250 on a meal for 2 though).
But if you really want to impress (and you should if you're trying to combine a birthday with valentines.... that isn't the way approach it... you want to oversell one but still celebrate the other....), rent a deluxe trapper's cabin at Baker Creek Chalets, about 20 minutes out of Banff.
You get a small, cozy "trapper's" cabin in the forest with a comfy queen bed, wood burning fireplace, jacuzzi tub for 2 (complete with surrounding mirrors), little fridge/part kitchen and bbq on your private deck (for breakfast -- bbq her bacon, hash browns, and scramble eggs on a griddle, serve with champagne and OJ, just before checkout) ...and all for about 300/night, comparable to Banff Springs/Rimrock/LL in price, but much more thoughtful.
http://www.bakercreek.com/accom.html#
Edit: Nevermind, didn't read the OP, just some of the stuff under. Baker Creek + Expensive Dinner = $600 night, and then some. But you could take her to the trapper's cabin and BBQ something, and spend the rest of the night in the jacuzzi..... 300 bones well spent.
