If you weren't planning on using the unit for anything beyond watching videos, then you don't need to do anything. Can't quite pin down with any certainty whether or not your specific video card supports hardware accelerated video decoding... It looks like the 4xxx series was where it was added, but it may have come just a little after the 4650, but it shouldn't matter. There should be ample CPU power to muscle through even high bitrate 1080p content with ease.
So, at most, just look for a cheap video card with hardware accelerated video decoding. Anything from the 5xxx series in the Radeon HD series should be a safe bet, and I think anything from the GeForce 2xx series on is also a pretty safe bet.