Hi Meszi,

I use the same SSL cert on both machines - you can install the same cert on the Ubuntu server and the SBS server.
Or you could use the SSL offloading setup where you have SSL up to the Ubuntu server and then HTTP on the internal communication (search google for front-end-https on).

With regard to RAM and CPU consumption, nginx uses very little anyway so concentrate on disabling unused services on the Ubuntu server rather than tweaking nginx - leave it running at init level 3 without full X server etc., and disable any unused services. Or get yourself a new server from the 21st century!!

Regs,
Cates.