Open up builtin ports FTP data and FTP download only on the packet filter side.
If you have a router / firewall with ACLs on it, like CISCO ACL, you might run in to issues with ACTIVE FTP. If you do, and want to keep your network secure and also be able to FTP stuff in and out, use a 3rd party FTP client that allows you to "choose" which FTP mode you start the FTP session in.
Apply ISA SP1 as well.
Shariq