There isn't technically a 2011 MP. 2010 was an update and only recently in 2012 was a tweak but it's pretty much the same thing. For cost sake I suggest getting 2008 MPs with the 8800 graphics cards. Or 2010 with the Quatro 4000s. Buy the basic ones. The graphics card is all that matters besides the SSD drives. Mac Pros since 2007 (with the x1900) have been running Catalyst fine. The 4870 on the 2009 I don't care for. This is why I say 2008 or 2010.
Snow Leopard works on all of them.