Low-side configuration are fine for simple loads like relays or lights, but you don't want to use a low-side configuration in this case, since it would place the ground of the SD card at a different level than the ground for the PIC24, which would interfere with the logical levels used to drive the data interface.
I like to use P-channel MOSFET's in a high-side configuration for this purpose, with the gate tied to the source lead through a 499K resistor to keep the device off when starting up. You can then turn the MOSFET on by configuring the pin on the PIC24 in an open-drain configuration and driving it low.
You want a P-channel MOSFET with a very low Rds (below 1 ohm) and low Vgs, such as this one. (I couldn't find any that met your criteria in a thru-hole package.) The MOSFET you chose (2N7000) has an Rds of 5 ohms, which means there would be a 1 volt voltage drop at 200 ma.
Here's a document that explains why it is best to use P-channel MOSFET's in a high-side configuration.