Stepper Motor Control Timing
The main question is what is actually going to handle the data coming over the wire?  The amount of crap (stuff that is not addressed to you) Ethernet can provide is staggering.  If you're going to use a low-end MCU (PIC24F), you're going to need some sort of co-processor, e.g. an ENC28J60, to provide MAC address filtering at the very least.

Someone in chat mentioned that the PIC24F has interrupt priorities, so if maintaining isochronicity is critical, you could just set the timer priority higher.

A minor aside: be aware that TCP/IP and Ethernet are non-deterministic (independently).  For example, don't expect to be able to send a command over Ethernet to stop your stepper motor "now" when you're right at position X and it to respond repeatably.