armondkoz is right. For transactions where the bank is suspicious about where the cash came from, it's $5,000. But it can be as low as $2,000 for suspicious transactions at certain "money services businesses" such as check-cashing outlets. For non-suspicious transactions at banks, it's $10,000. (It's possible the Patriot Act has lowered these numbers and the below web pages haven't been updated yet.)
From the IRS webpage on Currency Reporting - Money Laundering,
http://www.irs.gov/compliance/enforcement/article/0,,id=113003,00.html :
"Suspicious Activity Report (SAR): Filed on transactions or attempted transactions involving at least $5,000 that the financial institution knows, suspects, or has reason to suspect the money was derived from illegal activities. Also filed when transactions are part of a plan to violate federal laws and financial reporting requirements (structuring)"
Here's a quick-reference guide that has the rules for money services businesses:
http://www.fincen.gov/bsa_quickrefguide.pdf
Note that multiple transactions of smaller amounts can be aggregated by the bank, casino, or other financial institution to reach the level necessary to file a report. In other words, if you deposit a bunch of small cash amounts in a short period of time, they'll add them up and if it's over a certain amount they'll file a report.
As to what's a suspicious transaction, I don't know the legal standard, but common sense would say that if the 50 year old owner of a liquor store deposited $5,000 in cash into the account called John's Liquor and Grocery, it would not be very suspicious. But if a 22 year old wearing worn-out jeans deposited that same amount of cash into his personal account, it would raise eyebrows. Especially if it were multiple deposits.
And, to address the OP's question, if you look at the IRS page you'll see the word "transactions" used rather than "deposit." So even if it's just an exchange of $20's into $100's, the reporting requirements still apply.