You could check to see if your modem/router combo can support open-source custom firmware like DD-WRT or OpenWRT. The ASUS router you mentioned uses a version of this firmware straight out of the box. Installing custom firmware yourself can be tricky though, and if you make a mistake, you could brick your device. An easier solution would be to buy an inexpensive NAS device by QNAP or Synology. Two-bay NAS boxes use a lot less power than leaving your PC on to download stuff, come with PtP sharing clients for Bittorrent, eMule, etc., and have a lot of other useful applications either pre-installed, or ready to download from the manufacturer's store interfaces.