Erm BD movie watching or anything that has been encoded to other file formats can be watched on even a x1950 Pro AGP with the said above specs. Im not sure on my First rig because i dont own a BD Drive
Of course buffer size may be adjusted to minimum to have small and stable portions of data transferred and reduced latency. Despite, you'll have delays with processors video decoding -> less but more frequent fps losses.
Talking about high res video the bottleneck appears everything except VGA