Scott Baker:
50ft is the standard "limit" for RS232 (the +/- 10Volt level) granted you can acheive longer distances with slower baud rates and careful routing/shielding. You will not be 100% successful with that length @ 0-5Volt levels (I've read that even 10ft can be troublesome at high baudrates). Definitely go with RS232 or RS485. It's very cheap to implement. Or you can experiment with the 0-5V, but it may get frustrating with intermittant issues.