Hi Paul,
I haven't seen any Gig copper products for the PIX series, they've all been fiber based.
You will also struggle with throughput on the PIX520, its quite low. The PIX525 will do 370mbps (according to the docs) so should give more headroom for you, although it doesn't do full gig throughput like the PIX535.
Let me know if you have further questions.
Cheers,
Phil.