I am trying to find the smallest FPGA that implements 1 gigabit ethernet. I am planning to use the Marvel 88e1111 which implements 10/100/1000BASE-T. In the Spartan 3e datasheet Xilinx does not refer to the ethernet at all as it does in Spartan 6 datasheet. But here
According to that web page it shows both of these as compatible with the Spartan 3E.
Just open a project with the part you want to use then open the Xilinx core generator and find the Ethernet MAC and see if it's available. Generate the core and try to build it. I know the tri-mode mac used to come with a synthesizable testbench so you could build and test it on a board.