From what I recall, 14awg is sufficient for speakers level current at 50', so I imagine that's sufficient for line level. I picked up 12awg from
www.bluejeanscables.com for my speakers. Very nice cable and only 38 cents/foot. I've also heard very good things about wire and cable from Monoprice, especially given their prices.
You may have a bigger problem with ground-loops though, which is probably the source of the hum you're hearing. I never had much luck with using a PC as a source, and the difference in ground potential will be even greater if they're plugged into different electrical circuits. I would look for options that use a digital connection. Either a digital output from the PC, and using toslink or coax to make the connection, or an Ethernet solution (wired or wireless), etc.... I just wanted music, so I went with a Squeezebox to replace our CD player. There are similar devices that cost much less, as well as devices that transmit the output from your soundcard over ethernet.