Your CPU will bottleneck all of these cards, but there will be a noticeable speed increase nevertheless.
As for the drivers, I don't believe AMD's (who bough ATi) drivers are any worse than nVidia's. There have been issues with Vista, but I'm guessing you're not running Vista on your machine.
Jav400 and Feldon32 prefer nVidia though. (I have nVidia cards in my machine so you don't have to think I'm rooting for "my" team either
).
Paying €100 for that card seems a little short-term to me when you can get a new computer for less than €400.