Why would you even bother? An AGP2x slot can't even begin to give the GeForce3 a run for it's money. In other words, the AGP2x slot is a bottleneck for the GeForce3. The best card for the money is a Radeon in that AGP2x slot. Anything more and you're wasting your money.
That's the problem with changing the past. You never get to say "I told you so".
In that case, you could probably flash it. I personally have not done this, since I have a 4x AGP quciksilver. My suggestion wold be go for it. I would recommend the Radeon execept it's only 32mb. A Geforce 3 is the only 64mb card Mac guys have. good luck on flashing it.