Illuminated http://x10hosting.com/forums/payments.php (Make sure you select Illuminated and not Prime, Prime remains on the free servers whereas Illuminated is not on free hosting servers.)
If you buy Illuminated, it will be connected to your forum account so an email will be automatically sent to manually transfer that account to the Illuminated server.
If you buy Premium, after the transaction completes, simply log into the client area and submit a support ticket to transfer your free account to the new paid one.