two possible upgrades:
1: for insane resolution gaming, definately get a second X2. Normally putting that much GPU horsepower in a rig means it'll be CPU limited, but not with the i7. It's a perfect match, that will get you very real, very substantial FPS advantages.
2: go for an Intel X25 SSD + a F1 1TB for storage. Those choppy complaints are true, but Intel doesn't use the JMicron controller that causes it. Anandtech has an insanely detailed article about it if you really wanna read into it.
This upgrade will make the PC as a whole feel a lot more snappy and fast. I don't think you'll be really noticing the extra FPS you get from a second X2, but you WILL notice the speedy handling of your PC with an SSD, and it will add to the feeling that you really bought yourself something special