All you need to do to convert your 4250SX to a 4250TX is to remove the SX fiber adapter card. The TX interface is built in, so there is no need for an additional copper gigabit card.
There are plans to release an optional gigabit copper card for the 4235/4250 platforms, but it is only required if you want to run inline using 5.0 software. It should be available in the next couple of months.
-Rusty
