Jim, if you haven't already got it, go here
http://www.usg.com/Expert_Advice/const_handbook.asp?menu=
scroll to the bottom of the page, right click on the Construction manual download and "save as" to your drive - it's over 7 megs but a real time saver on answering questions about construction using drywall.
Your screw answer -
Type W (as in Wood)
Type S (as in Steel)
Type G (as in Gypsum)
The designator is what the screw fastens TO, not what it fastens. So for wood studs, a Type W screw is the one. For RC and Steel studs, the Type S screws are it. Type G is for laminating multiple layers of wallboard to each other, but should only be used if the wallboard is NOT on RC, and IS part of a sound wall.
For example, let's say you have a double framed wall between your control room and live room, and one side has the wallboard fastened directly onto the studs (since RC is supposed to be only used on one side, or it defeats the purpose) - Now, you want more than one layer of wallboard on the non-resilient side, in order to get a stiffer, higher mass wall for better bass isolation.
You can't just put screws through the first layer and into the studs when mounting the second layer, or the screws partially negate the first layer by flanking. So you need to mount the second layer without attaching it directly to the studs - this is where Type G screws come in. You spread joint compound over the first layer where the studs are, put the second layer up, and fasten it to the first layer OFFSET from the studs by about 2-3 inches and only fastened about every 12", this snugs up the second layer against the first layer, but DOESN'T completely laminate the two layers together (which would lower the coincidence "dip" and degrade the STC of the wall)
In your case, you'd use Type W screws to fasten the RC to the studs, overlapping the next piece of RC by 2-3" and using two screws at the overlaps - Mount the bottom row of RC with the long, open side DOWN, all the rest with the open side facing UP. Then fasten the first, 5/8" layer to the RC using Type S screws, making sure that you've marked where your studs are and NEVER put a screw through the wallboard where those studs are, only into the RC.
Then, you'd offset joints between sheets both vertically AND horizontally, and mount the second layer using longer Type S screws, again being careful to AVOID the studs; for each layer, leave 1/8" gaps all the way around the wall and caulk thoroughly before the next layer goes on. You should use 1/8" spacers under each piece of wallboard while fastening, then pull them and caulk before applying the second layer.
I think that's about it for now, I'll check every so often just in case you run into probs -
Be sure and download that Construction manual though - unless you've got broadband, it's a real PITA to wait for the pages you want and their search engine SUCKS - if you have it as a PDF on your drive, searching is quick and easy... Steve