I have an Nvidia GEForce2 MX/MX 400 graphics card with 64 MB RAM. For me this is more than enough and meets all my needs including playing a couple of OpenGL games. And I get decent frame rates with this card - provided I have installed the Nvidia's proprietary drivers for Linux. But to play the new games on the block especially the graphics intensive variety, you might need a more powerful card. And Nvidia and ATI are the two dominant players in the graphics card market.
Seriously how does one choose a graphics card from the ones available in the market? If you ask me, the best bet would be to strike the right balance between price and power. But there are a couple more things you have to consider before you make the jump.
James Yu lists 10 things one might consider before buying a video card. It is not something that one may not be aware of but nevertheless it makes an interesting read.