The only thing I've essentially tapped out is cpu, but I've just bought two x5690s to put in my main machine for £400. 12 cores @3.46ghz should be sufficient for the lifetime of the computer. everything else will be done over pic-e in future. and ram is getting cheaper every day, just bought another 16gb for my main for £55.All the while i can max out any game that i want at the moment. with a 780gtx in my machine it just screams. its expensive running a mac pro as a gaming rig sure, but it you game on the side and work on it mainly then its fantastic.
Great price on the CPUs, they should be about 250 by this time next year. How are you powering your 780 btw?