My favorite acoustic is also Bösendorfer, but not because it sounds great (though it does). It's the butter smooth response of the keys. There's really nothing like it.

Surprising your GF with a special gift when you know exactly what she wants, is great, but you don't know exactly what she wants. You could easily end up spending $3,000 + on a nice digital or used acoustic that is less than her preference.

While intending to demonstrate the depth of your love, the cost of the momentary "surprise" can easily lead to negative relationship stress. Prove to her that you respect her ability to choose what's best, above your own.

Either tell her before the holidays so she has a piano to play by the time the holidays come around, or (I would do the following) give her a piano jewelry box ( like this one) with a gift card inside that commits $3,000 towards a piano purchase.

Her joy of searching and playing many different piano's far outweighs any momentary surprise.
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Kurzweil K2600xs - yeah, it's purple :p , Alesis M1 Active MkII Monitors, Roland RD-300s Restoration Project completed.