OK, ho risolto.
Se a qualcuno interessasse ho fatto cosi':

<html>
<head>
<style type="text/css">
.nascosto{display:none}
</style>

<script language=javascript>
function changeIt(value)
{
if (value==1)
{
document.getElementById("testi").style.display = "inline";
document.getElementById("files").style.display = "none";
}
else
{
document.getElementById("testi").style.display = "none";
document.getElementById("files").style.display = "inline";
}
}
</script>

</head>

<body>



<form action="">
<select onchange="this.form.action = this[this.selectedIndex].value; changeIt(this.form.action);">
<option value="1">testi</option>
<option value="2">files</option>
</select>
</form>



<form name="testi" class="nascosto" action="">
<textarea cols=120 rows=15 name=testo>
</textarea>
<input type=submit value=Inserisci>
</form>


<form name="files" class="nascosto" action="">
<input type=file name=myfile SIZE=30>
<input type=submit value=Upload>
</form>

</html>