One of the best bang for the buck upgrades I've made in recent years is a running all of my software and Windows on a solid state drive. It's wickedly fast. Boot times and game loads are a snap, mere seconds. I have several large hard drives for the rest of my documents and video, which are slower but more more economical for storage.
I haven't had a need to upgrade my CPU or GPU in awhile. I've lost much of my interest in PC gaming and my current rig with an overclocked i7 950 and gtx 280 is more than enough for any of my normal usage. Back in the day, it was all about trying to squeeze a tenth of a frame per second more out of Flight Simulator, but I haven't even touched the game now in a couple years. The only thing I'm looking forward to game wise is GTA V, which has been delayed almost a year now for release on the PC platform.