Freakin prebuilt systems. i7 and GTX 680 paired with 1066mhz RAM.
Go into your BIOS and see if you can clock that RAM up to 1600. If not, spend a bit of money (less than $50) for 1600mhz RAM, assuming that the stock MOBO supports it. It'll give you a performance boost in an otherwise sound system.
Aside from that, adding an SSD is the only real improvement I can see.