And SSD is always a good upgrade. It would make loading programs much quicker. It won't do anything for games (besides making them load quicker) in terms of frames per second.
However what would you like to upgrade? What is your budget? What do YOU think you need/want?
You've currently got a fine mid range system. I really don't know what else to suggest besides an SSD.
If you do go the SSD route, the samsung 830 and Crucial M4 are good SSD drives.
I dunno. Is RAM an issue here or is it good to go?