It's not Graphics Card Power needed, It's a decent Processor and a large amount of Memory, as the Video is 2D not 3D.
I have one PC XP P4 2.4 ghz 2 Gig Mem XFX 6200 512mb,
My other PC Vista Ultimate Intel Core 2 Quad Q6600 @ 2400 MHz 4 Gig Mem XpertVision GeForce 8600 GTS 512mb (Lowest Subscore 5.6 3D Graphics).
The Vista PC is Slightly Quicker at rendering Video (HD or Standard DVD's).
A few bits to read click here click here click here