Hi FurryNutz, thanks for your attention. I got this working now! I'm from Brazil, my ISP is Net Virtua, my modem is a Motorola SBV5121 and I have a cable connection. I plugged my PC directly to the modem and, for my surprise, it didn't work either, so the problem was not related to the router. I contacted my ISP and they told me that ports 80 and 445 are blocked by default and if I want them opened, I have to pay $250/month for a 1 mb bandwidth connection. Since this proposal is absolutely ridiculous, I just configured Apache to listen to port 8080 and now everything is working just fine. I just changed port 80 to port 8080 in DIR-300 configurations, changed it in httpd.conf in Apache too and now everything is working properly.
Thanks again!