Or you can pick up and SSD, you wont see major improvements in game performance obviously, but load times of games and just overall quickness and smoothness of your OS is a major noticeable difference.
Host a lan party for your mates or take them out to the pub.
No seriously, 200 lbs would not result in a noticeable upgrade to your computer.
Maybe SLI another 670, but that would require another power supply, which puts you above budget.
A nice monitor or sound system?