Depends on your motherboard, or more specifically what type of AGP slot it has (1.5V only, 3.3V only or universal), and the period of games you wish the card to cover. 3dfx Banshee and Rage 128 series of cards are good enough for early 3d games, with the Banshee having the added bonus of Glide support. But both will quickly run out of steam when playing games past about year 1999 or so. For games newer than that, you want something with a lot more power, along the lines of a geforce 4, Radeon 9700 or newer.

There are so many threads on exactly this topic. Search is your friend.