I have used a flashed GeForce 3 myself. In fact, I'm selling it on eBay right now (
shameless plug).
Anyway, here's what you need to know about it...
1) It's about as fast as the Radeon 8500 for a lot cheaper
2) It's single-monitor only
3) The S-video doesn't work
4) It causes flickering in an AGP 2x slot, so your Quicksilver will be fine but a Sawtooth would be uncomfortable and difficult to use
5) It causes small horizontal lines to flash on the screen for about a minute after turning on your computer or waking it from sleep. These go away on their own, you just have to wait about a minute.
6) When enabling NV shaders in Halo, your view will get really messed up when using the zoom scope on any of the guns that have it. This affects ALL Nvidia cards, not just the flashed GeForce 3.
So, I just want to make sure you know the limitations inherent in it. I'll also be selling a Radeon 8500 of mine soon, because I recently got a Radeon 9800 and I'm just waiting for it to get to me. I have to say that there is almost no difference between the two cards. The Radeon is nicer in a few respects. It can work in AGP 2x or 4x slots, the horizontal line problem is not there, it should support dual monitors, you can adjust the clock speed, and it works with the ATI drivers so you can use application-specific OpenGL overrides. As far as speed is concerned, though, it's about the same.
By the way, you can sell your GeForce 2MX for $120+ on eBay. I suppose Cube owners love them. I upgraded my Mac from an original Radeon to that GeForce 3 and the difference was incredible. From what I've heard, the GeForce 2MX is pretty similar to the original Radeon, so you should be getting a nice upgrade if you get a GF3 or Radeon 8500.