http://forums.macrumors.com/showthread.php?t=1397367
slightly different but same model.
What changed? DDR3, Westmere Nehalem Xeon. But with 8-core 3GHz you are set and there never was much if any in cpu upgrades.
Keep your eye on the 8800GT of course, there are only a couple graphic card upgrade options but it will accept the same as newer Mac models. and keep the RAM (FBDIMMs) down to and below 70*C - monitor the sensors with Hardware Monitor (best) and boost the fans default speed from 500-600 up slightly to higher base of 800 rpm to improve airflow.
It is still a good solid unit. Protect it. Use a 850W UPS and it should serve you and run well. The ideal RAM configuration is to use all 8 RAM slots the combination of sets should not matter. 8x1GB or mix of some paired 2x2GB or you can use 2x4GB DIMMs as long as they are Mac Pro certified and have the large RAM heat sinks.