Skinny
There is a quick and dirty solution. Install the SSD and move the arma folder from its current location on the HD to the SDD. Then add a special link (called a junction point) so that the Windows and Steam think Arma is in its original location.
Details on the junction point can be found here:
http://en.wikipedia.org/wiki/NTFS_junction_point.
Its just a single command from a cmd prompt (with admin privileges).
mklink /J"C:\Program Files (x86)\game" "D:\Program Files (x86)\game"
Where "C:\Program Files (x86)\game" is the original arma folder and "D:\Program Files (x86)\game" is the new folder on the SSD.