Same here.
My specs:
Intel Core2Quad Q8400 @ 3.2GHz
RAM 4GB DDR2
Gigabyte GeForce GTX 560Ti SOC
Windows 7 64-bit

Showcase at 20-25 fps
Multi at 4-15 fps depends what kind of server
Multi on original missions 25 fps at beginning after a while it drops down to 12-15fps

Change from autodetect that is most high-very high settings to low gives me 1-2 fps more.

GPU 10-30 %
CPU 30-70 %

http://img716.imageshack.us/img716/1028/arma32013031623111817.png