Cricky,
I think either way, because of the nature of the ENC28J60, you will be lacking in Ram
I had to use sourcery to get 2 full ArtNet (or sACN) universes out to WS2812 LEDs
The way I did it is this
packetBuffer, stores the currently received packet from ethernet
Once I received it, and checked the right headers, universe, sub etc etc
I then jumped to the code to output to WS2812 LEDs
The code I used scanned the packetBuffer starting at [125] and output the bytes from there
This way I didnt need to use any other array for storing my values (the WS devices did that !)
I then went on to decode another universe, and output in the same way
(still not using any extra ram)
I wanted to carry on until I had decoded and output 2048 bytes, but I had problems with the ethernet chip buffer ram being full or something (my universe 3 was jittery !)
Unfortunately for you the DMXserial needs the second array to be in place so it can access it in the interrupt and output the byte(s)
I am still fiddling.....