man this has been asked a few times try searching the archives on this board. basically the best FTP service to run is called bulletFTP, if you are cheapo use IIS which is included with all windows server OS'es and winxp PRO. open your ports 21 & 20 for FTP services on your linksys router. i can almost read your mind, you must have a linksys BEFSR41 right?
192.168.1.1 in your browser will let you login your router config to do this. use a IP updater program on the server and sign up for your dynamic DNS service like no-ip.com or dyndns.org. whenever your dynamic IP changes the updater logs into your account and updates the IP. so all you need to give people is example: selene.dyndns.org instead of some cryptic quad number like 78.47.31.38 which changes every month or so. again check the archives