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It looks like you can't avoid the due diligence.

Part of the fun of buying toys is doing the research and knowing you bought what you wanted.
If the Tom Tom 700 and Nuvi350 are within $50 of each other, you have to get the Nuvi. The fact that it fits in a shift pocket is a huge selling point for me. The deal at Costco.com sounds plenty attractive. BTW - Costco.com also has an ad for the Tom Tom 700, but it currently shows as unavailable. I don't know what the cost was.
Did you see the Costco.com ad for the Lowrance350C which comes with a 4GB HD? They show unavailable as of today, but they did have units in stock last week with a selling price of $399. I played with one at Circuit City. It doesn't stand up to the Tom Tom 700 or Nuvi, but it's better than anything else in its price range.
Somewhere down the line, I'm sure some new portable convergence device will incorporate the functions of my Treo 650, my iPod, and my PSP, and a Nuvi350 and it's probably not that far away...
Have fun.
Regards, Mike