I went through the same thing when I was getting started. Like he said, everyone's very different.
I dive in 45F water, in a 4mm compressed neoprene suit. I wear only a 200 weight polartec suit and I am comfortable. I am not warm--I am cool, which is how I like it. I also use drygloves though they flood half the time.
My wife has a 6.5mm uncompressed neoprene suit which should be MUCH warmer and she still has to wear 2x as much insulation as I do to be happy. (Our dives are shallow too, so compression of her suit isn't much of a factor.)
I should also point out that we are 1-tank divers, with a max dive time of 50 minutes. The insulation we use might not be enough for long or repetetive dives. The othe divers in our area... I see them wearing what seem to me to be crazy thick puffy garments under their suits--but they are all more advanced that we are and I presume that they are geared for multiple dives in a day.
If you tend to be a person who is cold-tolerant, I'd start with a 200 weight polartec garment, otherwise start heavier as suggested above. I would also resign myself to trying a few things until I found the right combo... nature of the game.
Divesportsonline.com has a 200 g Tilos polartec suit for $75, and there are many affordable options on Ebay... there is a thread around here somewhere that discusses the ebay deals. Good luck...