I actually use the Garmin Street Pilot III. You can get them off ebay for under $400. Several nice things, especially if you are going to make a trip that long on the bike.
A) the unit is pretty small but does have a color screen and voice navi. (get to how this is good later)
B) it comes with a 128mb stick BUT can use other sticks, though they are only Garmin memory, you can get a 256mb stick for under $100 on ebay. I made it from NC to Colorado and up to SD on my 128mb stick. Of course I had the route planned and only loaded the maps for my route but that included ALL the data for the trip to that point. A 256mb chip would mean I didnt have to stop and load up anything for the entire route back home across SD, MN, IA, OH, WV, VA and parts of NC. Just something to consider if you are going solo on a bike, a smaller memory base means you have to take the laptop or have a way to load new maps on smaller units.
C) It autoroutes. Damn nifty if you miss a turn or something like that. You dont have to back track normally.
Now, back to the voice part of it
There's a bunch of devices out there that allow multiple audio sources into your headphones or helmet. Voice is nice because it will remind you when you are in town or closing in on a turn on the HWY. The speaker that comes with it probably wont work at hwy speeds but around town with the visor open it would work ok.
Also heard at cyclefest that there's a new product coming out that is going to be a wireless audio source selector. I think he said it would do like 4 different sources and send the audio up to the helmet wirelessly to some speakers. I'm on the list to get more info once its available, if you want I can send you a copy of it when I get it.
Hope that helps, I'm actually in the process of trying to figure out a good way to mount the GPS with a radar detector and my existing XM.