It depends on your system specs - cpu, ram etc. To be able to play it at high detail you really need at least the GeForce 6800 (or ATI equlivalent), 1Gb of Ram and 3Ghz processor. For medium detail you probably need 2.5Ghz processor, GeForce 6600 (or ATI equivalent). For lower detail I'm not sure as it gets buggy on a lot of the older graphics cards and you need to force them to run certain options which don't always work.
My latop won't run it well at all (to the point where I can't play it, way too jerky) and my specs are AMD Athlon 3400+ processor, 512Mb RAM and GeForce FX Go 5700 which isn't that shabby but I have to play it on my bf's comp :/
It's a very demanding game but if you need more in depth help I'd recommend you post in the B&W 2 board at
Lionheads forums. They're really helpful and will know exactly what graphics card you need for different levels of play (and you will get help very quickly and just ignore the New Member 's) - my bf is a regular there
Just explain you need to update your graphics card.
I hope you get it sorted soon - the game is absolutely awesome
EDIT - this thread might help you =)
http://allboards.lionhead.com/showthread.php?t=99676