Well you have a lot to consider. Begin with the fact that we are all different in make-up and yolerance to the cold. lisa_j made this very clear.
Next there is no free ride in the deeper lakes. My coldest New England summertime water has been in Lakes and quarries, down in the low 40sF. The neat thing about lakes is the abrupt thermocline. You may spend part of the dive in very cold water but as you work your way back you will re-enter warm water and have some chance to recover before surfacing. Ocean thermoclines in my opinion have not been as dramatic. On shore dives I will sometimes run into an off shore wall of cold where the open ocean meets shore warmed water.
Many begin to hit their "wet limit" around 50F or a little less. Here in Maine that got us almost to Thanksgiving. You can still dive but the surface interval and dress down will become challenging and you may not have enought body neat to safely make a second dive. Again it's all personal. The topside stresses often are more daunting than the water chill.
The most common configuration for a long time has been a 7mm farmer John (or Jane) set. This is basicly the bib overall and a long sleeve jacket that goes down to mid thigh. This provides 2 layers (14mm) on the torso, groin and upper legs. You end up with pretty good freedom of movement and good protection. With this a hood with a tucked in bib is usually worn. Some peoplke like the John/Jacket set because they can use the pieces seperately, IMO they work as a set and seldom fit correctly enough alone to be good for much.
Let's stop for just a second and talk about wet-suit 101.
*It needs to fit like a second skin
*The water volume trapped wants to be as little as possible
*The water wants to stay in the suit, not get exchanged for more cold water.
*It must allow good freedom of movement and must not restrict your breathing.
*Fit is everything.
I prefer to start with a 7mm full suit. This is a 7mm jumpsuit with full arms and legs and a back zipper. Here is an example:
http://www.bare-wetsuits.com/bareshop/diveproduct.asp?dept_id=23010&pf_id=61424
Lets assume that you will need head protection for any diving though you may have some freshwater dives above the thermocline where this is not the case. The biggest weakness of a full suit is the back zipper. Some water will weep in and you back can be very sensitive to this. Next problem like any suit is the neck opening. Protect your head, mute the back zipper leak and vitrually remove the neck opening from the equation with a hooded vest like this:
http://www.bare-wetsuits.com/bareshop/diveproduct.asp?dept_id=28520&pf_id=61437
One nice thing about a vest and full suit is that you get 2 good layers on the shoulders which are your cutting edge. Some Johns have fdairly narrow shoulder straps.
I was very comfortable with those 2 items all summer. In the fall as temperatures rolled off I switched to a heavier vest:
http://www.bare-wetsuits.com/bareshop/diveproduct.asp?dept_id=23010&pf_id=61426
My last pair of wet dives were in the upper 40's and I was fine. I did buy a drysuit after that but I could have kept diving wet for a while still.
I do have 3mm & 5mm suits as well and mix and match them with the vests mentioned above. I do both skin and scuba dives so my conditions and needs vary a lot.
http://www.bare-wetsuits.com/bareshop/diveproduct.asp?dept_id=25010&pf_id=61435
So there you go, this is what works for me. I have had surface swims on lakes in the low 80s and gone down into the low 40's 60 feet below!
Have fun,
Pete
