What about just going with the gift you were going to buy?
If she's cool with your gift, then you know you got a keeper. In that case, she obviously doesn't care how much you spend on her; "it's the thought that counts, not the price".
On the other hand ....
If you start getting the Evil Eye afterwards because you didn't break the bank to out-do her Xbox 360 gift, then ... well, where's that eject pic ... it's a trap ... where's Wedge ....
Because if she expects you to always one-up her on gifts, that's just gonna snowball. First it's V-Day. Then it's her b-day. Then it's Xmas. Before long, you are enmeshed in a constantly escalating price war where no one can win and you BOTH lose.
So I say to you: don't automatically assume she's got a bullet with your name on it. You've been together for 4+ years, you oughta know her (fairly) well by now. Go with what you were gonna do and see the results. If the results are bad, then thank your lucky stars that you pulled the ripcord before the big M. But don't automatically assume the results will be bad.