Okay, now that I'm not at lunch, I can go into this a bit more.
Firstly, it's helpful to know the rest of your system, to see if it's even worth upgrading your card. Going off just what I know about your existing card...
You're more than likely running AGP, which is a dying technology. If you want to stay AGP, this is going to limit you. You're better off getting a PCI-Express motherboard and card, which will set you up for the future.
However, there's no way in heck you'd be able to do this for $100. In fact, it will be difficult to get a better card for $100; and AGP variants of existing PCI-E cards are more expensive than their PCI-E brothers.
Are you willing to spend a little more on this? If not, buy the ATI X800GTO (~$169). It's the cheapest option with good performance on AGP.
If you are? Get a PCI-E motherboard (~$100) and either the PCI-E version of the 6800GS or a 7600GT (~189).
eVGA makes good nVidia based cards, and both Sapphire and ATI make good cards for ATI chipsets.
http://shsc.info/PCPartsPickingGuide is a great source for PC parts info and is where I gleaned a bit of my knowledge for this post.