I wouldnt consider SLI at all, its too expensive compared to the performance benefit.
The 5770 would be a cheap upgrade and would give a substantial boost to graphics and FPS, and it costs around £100. Remember to reinstall Windows (format computer, take backup first) when you do this as problems can occur when you change GPU-vendor without reinstalling windows.

SSD-drives are nice if you can afford them and your PC runs WoW fine and you just want to speed up loading.