The maximum power the GN will draw is 1A & buying anything larger than that isn't going to gain you anything.
Also, there is a difference in what the GN will draw from wall charger vs a USB port. A PC's v2.0 USB port will only push 500mAh & when the Galaxy Nexus is plugged into a USB car charger like this one:
Amazon.com: Scosche Dual USB Car Charger for iPhone 4/4S, iPod, MP3 Players and More: Cell Phones & Accessories
It sees it as a USB host and correctly draws the 500 mAh amount. If you have patience, a soldering gun and some duct tape; you can rig solder the chargers 2 internal data pins together and the phone will no longer see it as a USB host. 500 mAh is not enough to charge, run GPS, and pull map data at the same time. You will probably see battery levels drop instead of go up. When through that for years until I did a little research.
Also, keep in mind that the dual port chargers, like the one I linked to above may say they are 1,000mAh, but it's actually 1,000 mAh shared across both ports. Each outputting 500mAh.
Car chargers - on the other hand - do not look like a USB host. They look like a wall charger to the phone. So, the closer you can get to 1A the better. The Verizon
car charger outputs 850mAh, but this one outputs 950mAh:
Amazon.com: Motorola Vehicle Power Adapter micro-USB Rapid Rate Charger: Cell Phones & Accessories
I highly recommend the $6 investment. I've recommended this charger in almost every forum -- including this one & as you can see by the number of ratings it's received -- a lot of people like it. Avoid knock offs and make sure you're ordering one that is fulfilled by Amazon. That was the only common complaint in some of the reviews.