The question the OP needs to ask is if the ~$40 is best spent on extra RAM?
There is no downside from upgrading to 8GB - it'll never make your computer slower, and even if it doesn't speed up your current set of applications it may speed up a different application in the future.
For your intended uses, upgrading from 4 to 8 GB will result in minimal, if any performance gains. HTPC for media playback usually means ~1-2 process currently running, and AFAIK media center/VLC don't really use more all that much RAM for standard playback. If you want to use custom filter (like ffmpeg) to enhance your media, upgrading might help slightly in these instances.
"Light" gaming also seems to imply a similar scenario, where ~1-2 process are actively in use. Most games can't use more than 2 GB, since they are compiled a 32bit programs, so upgrading will result in negligble performance gains.
Your system will likely "feel" a bit more responsive, since windows can use extra RAM for caching your file system, making programs you launch often seem more responsive.
TL&DR - 8 GB won't offer any significant performance boost for your intended usage. Your system will probably feel slightly more responsive due to file caching. You should decide if future proofing + a slight performance bump is worth the money to you.