$50 is fair for a 500GB drive, if you need the space.
1GB of video memory isn't needed with this system with this low of a resolution, even if you were to play games. 512MB is more than enough.
You cannout output 5.1 audio through the headphone jack, as that is analog and can only carry 2 channels. You need to check if the notebook has a digital audio output. If it does, it will be in a 3.5mm version, and you'll have to buy (or make) an adapter to work with a standard coaxial cable.
The GPU may or may not allow you to output audio through the HDMI connection, but if it does, your receiver should be able to use that just fine.