That's actually a great question, with a simple answer - do the GPU and SSD. A GPU will only give you so much more performance in games, and going from a GeForce 8600 GTS to even a GeForce GTX 660 Ti will gain you some serious performance improvements in every single game you play.
An SSD, will improve every single application on your computer from Windows, to your web browser, to your game load times. I would definitely suggest grabbing the mid-range GPU and a 240GB SSD.
Once you use an SSD, you'll never want to go back to a mechanical HDD for Windows/Games.