I wouldn't spend it on anything performance related because that build is in a nice little sweet spot where $100 wouldn't get you much more.
- If you don't like that Silverstone case, you could get a different one?
- If you care about quietness, you could spend it on good quality fans?
- Maybe a good quality headset or some speakers if you don't have them already?
- You could spend the extra on smaller components to make a much smaller build if you care about size?
- You could add more/bigger hard drives if more storage would be useful to you?
- If you don't already have a good mouse and keyboard you could spend it on those?
If you want any specific suggestions on any of these things, let me know.