Hola:
Ejemplo:
<form action="prueba.html" method="get" >
<input type="text" name="dato" value="a" />
<input type="radio" name="r" checked onclick="this.form.action = 'prueba.ktml'" />
<input type="radio" name="r" onclick="this.form.action = 'prueba2.ktml'" />
<input type="submit" />
</form>
Saludos