I'd rather add a second HD 5850 than the SSD. SSDs are still too expensive to be attractive, and don't really do that much other than the feel of using it. i wouldn't consider adding one until the 120 GB are around $150 (or lower). Unless you're doing a lot of rendering and encoding or such, more RAM isn't going to do anything for you. RAM cooling is completely useless.
Of course, adding the second 5850 might cause issues with that crappy Coolermaster PSU. Might want to consider getting a good one, like XFX's 750W for about $100 (modular, 80+ Silver) or Antec's Earthwatts 750W (non-modular, 80+ I think). Those would be more worthwhile upgrades than more RAM, RAM cooling or a SSD.