Don't worry too much about it, Radagast.

Just remember that in AD&D 2nd Edition, and Baldur's Gate, the formula is, as you said, THAC0-AC=what you need to roll to score a hit. THAC0 is actually even MORE important than AC, because you should try to be attacking the enemy more often than you're being attacked.
I'm told that in the 3rd Edition rules, BOTH numbers were changed around; a positive AC is good, and a positive THAC0 is good. Don't ask me for any formulas, though.
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Boots of Stealth: 1125 gp
Shadow Armor: 7875 gp
Slipping an Oil of Firey Burning down the back of Sarevok's armor: Priceless