Marc
Fast cards have never been cheaper . You can get Nvidea series GT240 cards now for around $100-$110, which will be much faster than any on board graphics accelerator. They may help with screen redraws as you are tabbing through your windows. I wouldn't spend much more than $100 if you are not seriously into gaming. That being said my `old' Nvidea 9600GT still seems to handle any game I've thrown at it quite comfortably. I simply can't see how a $400 card would give you any advantage.
Heres an idea of some of the prices ( Beecom are my favorite benchmark ..my local computer guy will always match their prices if I quote them )
http://www.beecom.com.au/home.php?cat=407