My suggestion if you don't want to spend alot of money on the gps itself is to buy a RAM mount. They are seemingly waterproof and come in various sizes so you can buy a standard automobile gps and throw it in the RAM mount when you're on the bike. Mine works well and water hasn't been a problem. However, using a power cable means punching a hole in the back and that's where the waterproofing becomes questionable.

My suggestion if you don't want to spend alot of money on the gps itself is to buy a RAM mount. They are seemingly waterproof and come in various sizes so you can buy a standard automobile gps and throw it in the RAM mount when you're on the bike. Mine works well and water hasn't been a problem. However, using a power cable means punching a hole in the back and that's where the waterproofing becomes questionable. I haven't done that yet so I can't comment on how well it works. Since I bought mine about a year ago, maybe they've come out with some new products to address that issue. I think it's worth looking into.

I dont know what ram mounts you have seen but my ram mount is open except for where the GPS unit clips onto it. and now my wife made me forget my train of thought. I had more to say but cant remember it now.

Here's a RAM case that holds a GPS and is water proof:
http://www.ram-mount.com/aqua_box_mount/aquabox_medium_mount.htm

I use my Garmin Zumo 550 on both my bike and in my car. Since it is designed for motorcycle use it comes with a mc mount, but also a car mount with built in speaker. It does have text to speech. It will also play mp3's off of an SD card. It will integrate with a bluetooth headset and your bluetooth compatible cell phone. It biggest drawback is the price. Around $700.

Doesn't anyone use a mapbook anymore?