Steve, what you are describing is called "table stakes" and it applies in all public card rooms, regardless of the game (hold'em, 7-stud, etc.)

When you were allowed to call $5 of the $8 raise, you were actually only calling the first $5 of the raiser's money, not his additional (uncalled) $3. If it were only the 2 of you playing, the raiser would take back his extra $3, and you would both show your hands.

If there were others in the hand, they would still have to call the full $8 (or fold). The extra $3 from the original raiser, plus $3 from each other caller of the full raise would form a "side pot" consisting of $3 x no. of other callers excluding you. Example, in 4-way hand, you are all in for $5, and other 3 are in for full $8. The dealer takes $5 from each hand and puts it in the main pot. Then he takes remaining $3 from the other 3 players ($3 x 3= $9) and puts it in a new "side pot". The highest hand of all 4 wins the main pot, while the highest hand of the other 3 players wins the side pot. You cannot win the side pot, because you have no money in it.

When you put your last money into the pot, it's known as being "all in", and from that point on, you cannot be forced out of the pot (for lack of money), but you cannot participate in any additional action beyond that point, even if there are multiple raises and reraises after you are "all in". I've seen cases where multiple players are all in during the course of a hand, and there are several side pots. Don't worry, the dealer keeps track of which side pots apply to which players.

To answer other questions, after a player goes all in, he must purchase a stack of chips at least equal to the minimum buy-in for that size game. Say, $40 in a 4-8 game. That way, a player can't always play on "short money" in front of him. Occasionally, a poker room's rules allow for a half-buy in (a "short" buy) every other time. But most of the time you WANT to have more than the minimum buy-in, so that if you do get a great hand, you don't run out of chips. Think of the agony of having a straight-flush and only a few dollars in front of you.

To summarize , you can add dollars to you stack anytime BETWEEN hands, but not during a hand. Also , you cannot take money OFF the table, except to toke the dealer or cocktail server, until you are ready to leave the table.

(I'm turning in for the night, but will try to clarify further tomorrow if needed. See you at the tournament.)