For the past few months, I've been back into training in the gym and re-realizing what muscles help the golf swing, and at what points. I've managed to boost my average SS by around 5 MPH since last summer. Here's what I've found so far.

Work legs - squats for quads and glutes, calf raises - helps balance tremedously... allows me to take a bigger rip without fear of balance problems. I can't say I notice a gain in what some people call "leg drive", but balance is definately easier with a more stable base.

Abs and lower back - ab roller, sit-ups, hanging leg raises for abs... good mornings and deadlifts for back - Strengthening these muscles and losing a little flab has given me better posture at address and more torque from the midsection through impact... and I'm back in size 30 pants!

Lat muscles and pecs - pulldowns for lats... bench press and dumbell flyes for pecs - there is definately a significance with these muscles in the transition from the backswing to downswing and in the entire area of the swing when your hands are above your sternum. Creating tension between these two muscles starts the power that transfers to the torque created by the abs and lower back later in the downswing.

Forearms - wrist curls and rev grip curls - tremendous advantage when hitting from a bad lie (rough, buried, etc...). I think the actual importance for you would be based on the amount of wrist action you utilize through impact. I've been trying to be less reliant on flash speed created by snapping my wrists through impact, so I don't think this has made a huge difference for me. I recommend stretching if you're going to build your forearms... mine are always rigid and in need of stretching to prevent a painful pull/sprain/strain.

I haven't found any major relevance in shoulder strength or arm (bicep or tricep) strength, nor from the muscles in the upper/middle of the back or trap muscles.