Password Encryption with Custom Membership Provider

 I am using a custom membership provider with a custom ValidateUser method.  The ValidateUser sends and additional parameter to authenticate my users (Username, Password, and Dealer).  I created a custom stored procedure for ValidateUser to call.  I copied over all my users from another table and encrypted all the passwords in the aspnet_membership table using the code below.  My question is, how do I take the password the user enters in the login form and validate that against what is in my aspnet_membership table?  Is there a method I need to call to encode/decode to do the password check?  Thank you very much for your help!
Here is the code I used to encrypt the passwords (not even sure this was the right way to encrypt. Please tell me if I did this wrong):
public static string EncodePasswordNow(string originalPassword)
    {        
   & ...