If an employee is a "salaried" employee and still receives overtime pay, what is the law regarding that pay? (The employer and employee agreed ahead of time that anything over 50 hours would be overtime). Is there a standard rate of pay for overtime and does this apply to salaried employees?

"Salaried" and "hourly" are just payment methods and do not mean much by themselves.

"Exempt Salaried" employee never have a right to paid overtime and there are no rules on paying overtime to Exempt Salaried employees. Federal DOL has indicated that doing so does not risk the Exempt status. Court decisions have been less clear. A minority of court decisions seem to think that counting actual hours worked and paying overtime based on actual hours worked to a Exempt Salaried employee risks the Exempt status. A majority of court decisions do not agree with this position. And of course, states rules are whatever state rules are, and the states do not agree on much of anything (those states that actually have opinions on this subject - some do not). Many payroll people suggest that if the employer pay Exempt Salaried employees overtime, that they base it on something other then hours worked.

"Non-Exempt Salaried" employees mostly are paid overtime based on the same method as Non-Exempt Hourly employees. All hours worked past 40 in the workweek are paid 150% of Regular-Rate-of-Pay for hours worked past 40 in the work week. This are for those Non-Exempt Salaried employees covered by the 29 CFR 778.113 rules, which is the large majority of such employees. There are also a small minority of "Non-Exempt Salaried" covered by the Fluctuating Workweek or Belo Plan rules, which are somewhat different.

Since you specially mentioned Exempt Salaried in your title, I am going to assume that is what you are looking for.