http://www.anandtech.com has a price guide they post every once in a while. From that, it looks like you have either the option of going for an ATI 9000 (128 meg probably better than the 64 meg version and it's only about $10 more) or saving up a little more and spending about $130ish on a GeForce 4 4200. If you can wait a little longer, the GeForce 4 card prices will probably drop quickly as soon as the GeForceFX is released.
I would avoid the Nvidia "MX" cards like the plague. They're essentially re-branded GeForce 2 cards with various memory architectures.
You could go for a used card... A nice cheap used card would be a GeForce 2 GTS or Pro with 64 meg, and it would run AH and most other games just fine. Shop around some, just don't buy an MX card unless you absolutely can't do anything else. The GF4-4200 is a very good card for the price, and you might even find one on sale.