Well lets assume you're playing a HD video at 1920x1080. Uncompressed, thats 1920x1080x24/8 = 6.22MB's per frame. Video typically plays at 24frames/sec which means at any given second, you're outputting about 149.28MB's of data which your video card has to process.
Like the previous poster said, if you factor in that you're playing games with AA/AF at high resolutions you might reach the RAM limit of your GPU. However thats quite unlikely unless you're playing something like Crysis at 1920*1200 at super high settings with 4xAA/AF or something like that.