radio button select/enable text area and validate input is a #

Hi there,

I have a pair of radio buttons, one with a text field. I'm trying to figure out how to
  1. enable/set focus to the text field when the button is selected    and
  2. verify that there is info in the text field   and
  3. verify that the info is a number. It needs to be a whole # or limited to 2 decimal places (ie a monetary value)

Here is a snippet with the buttons...

<?php
        if ($deposit == "0"){
         echo "<input type='hidden' name='payment' value='".$page_price."'>";}
        else {
         echo "<tr>
       <td width='160' height='25' valign='bottom' class='forumheader2'>Payment options :</td>
       <td width='410' height'25' valign='bottom' class='forumheader3'><input type='radio' name='payment' value='".$page_price."' checked='true'>Pay in full</td></tr>
       <tr><td width='160' height='25' valign='bottom' class='forumheader2'></td>
       <td width='410' height='25' valign='bottom' class='forumheader3'><input type='radio' name='payment' onClick='other.focus()'>Deposit :  $<input type='text' name='part_amount' class='tbox' value='' id='other'></td>
      </tr>";
      }?>

And here is some verification code I have already which checks some other text fields later in the form....

function formCheck(formobj){
      // Enter name of mandatory fields
      var fieldRequired = Array("firstName", "lastName", "address", "location", "state", "postcode", "phone", "mobile", "email");
      // Enter field description to appear in the dialog box
      var fieldDescription = Array("First Name", "Last Name", "Address", "Suburb", "State", "Postcode", "Phone #", "Mobile #", "E-Mail");
      // dialog message
      var alertMsg = "Please complete the following fields:\n";
      
      var l_Msg = alertMsg.length;
      
      for (var i = 0; i < fieldRequired.length; i++){
            var obj = formobj.elements[fieldRequired[i]];
            if (obj){
                  switch(obj.type){
                  case "select-one":
                        if (obj.selectedIndex == -1 || obj.options[obj.selectedIndex].text == ""){
                              alertMsg += " - " + fieldDescription[i] + "\n";
                        }
                        break;
                  case "select-multiple":
                        if (obj.selectedIndex == -1){
                              alertMsg += " - " + fieldDescription[i] + "\n";
                        }
                        break;
                  case "text":
                  case "textarea":
                        if (obj.value == "" || obj.value == null){
                              alertMsg += " - " + fieldDescription[i] + "\n";
                        }
                        break;
                  default:
                  }
                  if (obj.type == undefined){
                        var blnchecked = false;
                        for (var j = 0; j < obj.length; j++){
                              if (obj[j].checked){
                                    blnchecked = true;
                              }
                        }
                        if (!blnchecked){
                              alertMsg += " - " + fieldDescription[i] + "\n";
                        }
                  }
            }
      }

      if (alertMsg.length == l_Msg){
            return true;
      }else{
            alert(alertMsg);
            return false;
      }
}

Any assistance with this would be greatly appreciated, as my JavaScript skills are practically non-existent. I have pieced all of the above together by searching the net and am now a bit stuck.

Many thanksStart Free Trial