I think that the memory on the video card itself is most important when playing on a screen that big. Try to find a card with 1GB of GDDR3 or better. Do not buy anything with GDDR2.
An ATI 4670 1GB should be fine. You can go up to an ATI 4870 1GB if you want to, but you wouldn't need it for just movies. The 4670 1GB is only about $90. You could do alright with a 512MB card if you want to save a little money.
(All these cards are for a PCI-Express 16x slot. For AGP, your best choice is an ATI 3850 512MB for $120+.)