KnightFighter: I think some card games would be hard to play on a turned based game site, but for example

Black Jack - it would be easy to hit/double/split - have up to 10 other "real" players playing - then after all 10 make their move (within the time limit) - the "dealer"/computer plays against the ones who did not bust. [Each person starts with a set amount of coins - out of coins, lose - or possible play a set number of rounds with the person with the most at the last round wins.]

Texas Hold 'em, Poker - Just go around - not too many notes would be needed.

Rummy - simple enough, no notes would be needed.

Some others might be a little more difficult, but possible some small "tweaks" to the rules where players play against the "house/computer" instead of against other live people could work for some games.

For games where what the other player has in their hands can effect others, figuring out how to handle time outs will take a little planning.