At the moment the RadCaptcha control can validate only asp:TextBox. You cannot validate the input from a RadTextBox. If you want the asp:TextBox to visually look like the RadTextBox you can decorate all of the TextBoxes on the page using the RadFormDecorator control.
RadCaptcha performs only server-side validation. However you do not need to turn off client-side validation of all other asp:Validator controls on the page, as the RadCaptcha will validate the input whether or not client-side validation is turned off/on.
the Telerik team