http://www.anandtech.com/video/showdoc.aspx?i=3678&p=2
According to Anand's tests, GPU and the new flash make a huge difference.
Remember to buy the NEWEST video card, not the fastest. A GeForce 8800 is a fast video card, but it won't help if it doesn't support the features you're looking for. The newest cards will have more features, better GPGPU support, and better decoding support.
Nvidia's flash GPU list is
here. It looks like GeForce 8 and newer support flash.
AMD's flash GPU list is
here. Looks like discrete cards must be Radeon 4xxx or newer.