I do not know if you have an SSD, but in my opinion an SSD would be a very rewarding purchase, and great upgrade. Faster boot time, overall better performance when using your computer, and faster load times when playing games.
Well I do think upgrading to an i5, but you already took that off the table, maybe a gpu?
I like the idea of upgrading to 7950 but keep in mind that broadwell will not be miles ahead of ivy bridge. So upgrading now won't make that much of a difference later.