The Mac Pro (Early 2008) 3,1 does have 64-bit EFI as the newer models, so technically it should be supported for some time to come but it is over 7 years old now, so no idea how long Apple will support it with future OS X versions.
Even though I have an 3,1 model I would probably invest the extra $$ and get the 4,1 model, which can easily be upgraded to the 5,1 model firmware and use modern processors and less expensive memory (FB-DIMM is still somewhat expensive if you can find it for the 3,1 model).