I have the Garmin eTrex Legend. It's a small, light, and cheap (bought for $129).
It has a monochrome screen but has a backlit function for nights. As for navigation and trip planning, you can do it either on the unit itself (which is limited) or hook it up to a laptop with the included serial cable. I use it with Streets 2004 and it works great. My laptop screen is so much bigger than any built-in GPS system. The only thing that's missing is the voice nav. feature. For that I'll have to wait for my next car.