@ chafari, your outdated mate, Max 9 has a symmerty modifer that does all the work for you.
Ohaio Zeus,
This way should make life esaier for you, Clcik on the mesh your using and go on the modifers tab. After a little searching or press S youll find the symmertry modifier.
This mirrors the mesh for you and closes the seams, ' welds the polies together to create a solid mesh.
I have poorly put together a pic of what i mean in the DL.
Anyway, since you only want half the mesh mirrored, click the drop down box on symmerty, then clcik the mirror sub function.
you can now control the area which you mirror by moving the mirror line along the axis of your choosing. to change the mirror axis, Look at the parameters below the objects modifier hierarcy and choose appropretly.
See I am smart, i just act stupid.
Obvious typos, but im meant to be working so im typing fast.
If you need any other help, just ask.
-Con