First off,
Welcome Pete.
In my personal opinion there are only three legitimate grading companies for prewar issues (SGC, PSA and BVG). Each of these three tend to follow reasonably similar grading standards.
Again my opinion is that its better to collect the card not the plastic slab that surrounds it. I like to buy cards that are 'raw' (aka unslabbed) because they tend to be cheaper that way. However for nice conditioned items like (ex/5 or better) it's usually safe to buy them in a slab so that you know that the cards are less likely to be trimmed or altered.
Common PSA 3s will tend to sell for about $35 each. Some cards will be more either because of their scarcity, popularity of the player or the brand of back they contain (those without sweet cap and piedmont are more desirable).
As for sale cards I have roughly 200 duplicates ... almost all are raw and most are roughly "Good / 2 condition" --- even though some are better. Shoot me a line if you want to know more or have other T206 questions.
scottglevy@yahoo.com