What you will probably need is the 11-2 ATI series driver. However, I would go with the 11-1 series instead, as 11-2 created some issues with other legacy games.
As for nVidia - I have the latest nVidia drivers and experience no issues with NWN. According to fraps I get an average of 300-350 fps while moving through low impact areas, which drops to 275-325 fps in high-impact fights (lots of VFX running and lots of creatures). The worst fps I have gotten is 220 - and that dropped only happened cause my PC started to backup files in the background.
I use an nVidia GT 240 GPU with 1GB VRam. Although an older card, I highly recommend it if you are going to buy a "real" card. It currently retails for less than $100 and is a bargain compared to some of the comparable ATI cards.
I really can't recommend anything from AMD/ATI. I dropped them over a year ago and went back to Intel and nVidia - the only hardware I've ever owned that lasts longer than a year.
