There are a lot of snazzy and very cheap micro sd card reader/writer devices that'll hook up to USB. I'm guessing like, radio shack and five dollars. So unless your phone is a weird one that won't play a sound off the card, this process should let you use any video game sound/song as a ringer:
1) buy a microsd reader/writer
2) download a song off the internet or snag it from emulation or what have you
3) copy the sound to your micro sd
4) put it in your phone and select the song as your ringer
If your phone can do MIDI, there's a huge collection at vgmusic.com. You can find some good remixes at
www.ocremix.org. Also, you might be able to torrent the soundtracks to more popular games off of a major torrent site.