You can put a Westmere (6-core) 3.33GHz chip into a 2010 model 2.8 or 3.2 quad, it will accept the upgrade. Simple chip swap, use hex wrench, take of heatsink, take out chip, put new chip in, apply thermal paste, screw heatsink back on.
To upgrade a 2009 model single processor Mac Pro (2.66, 2,93, oe 3.33 quad) to Westmere, you will need to do a board swap with a new backplane and processor board from a 2010 so you have the correct firmware on your boards to accept the B1 stepping of the Westmere chip. The firmware built into the 2009 Mac Pro boards will only accept Nehalem chips with D0 stepping. Roman23 has upgraded his 2009 Mac Pro with 2010 boards, his System Profiler reads "MacPro5,1" and he is ready to accept a Westmere chip in his machine. The upgrade cost him about $400 for the boards, he got it done for really cheap.
You can't put a Westmere chip in a 2009 model, the stepping won't allow it. It won't even boot. The Westmere chip is B1 stepping which is different from the Nehalem chip's D0 stepping. The MacPro5,1 has modified firmware to accept BOTH D0 and B1 stepping chips (the 5,1 can take BOTH Nehalem AND Westmere chips, yes.) Roman23 is running his Nehalem 3.33GHz chip on his MacPro5,1 board and it works fine. It's D0 stepping. He's waiting to save up enough money so he can buy a Westmere chip to do the upgrade to 6-core.
You will need the 2010 Mac Pro System discs if you upgrade a 2009 machine with 2010 boards. The new 10.6.4 that ships with the MacPro5,1 is modified for the boards (has something to do with the system bus) and the system won't run at full speed unless it's running the new Snow Leopard build designed for MacPro5,1. Upgrades 10.6.5 and later should work on any Mac without an issue, but you will need the newer Snow Leopard build (NOT 10F569 but the new one) to work with the MacPro5,1 boards. Roman23 ran Geekbench on his upgraded Mac Pro running the 10F569 build and was only getting about 5200 scores, he upgraded with the 2010 system discs and was getting 11700 scores. Clearly something up there.
So, in conclusion...you can upgrade any 2010 Mac Pro with any Westmere or Nehalem chip that is either D0 or B1 stepping, but you can't put Westmere in an existing 2009 Mac Pro, you'll need to do a 2010 board install (both the Backplane/Logic/Mainboard and the Processor Board, so the firmware is matching). I can confirm that it's been done and it works.
Good luck!