It was some time ago, and unfortunately I can't remember all of the details. But, I replaced the supplied antennas on our SG565 with two 7dB ones and it made a huge difference to our wireless coverage.
As far as I can recall, the connectors on the SG are the most common variety. I ended up using exactly the same replacement antennas on my DLink wireless access point and a Linksys wireless gaming adapter/bridge at home - they all had the same fitting.
On the strength of the description in this link alone, which includes the model number of my DLink access point specifically, I'd suggest that the fitting you are looking for is an RP-SMA:-
Hope that helps.