I don't think you can get a decent projector under $400 so I'm going to recommend the iPad1 (the only difference between iPad1 and iPad2 is that the iPad2 has better cameras for video recording).
Laptops can be nice. It's nice to have an actual keyboard, and the monitor makes a mini-DM screen. But they also takes up a lot of tabletop space and the heavier weight makes them more of a hassle to move. And laptops usually have much shorter battery lives than e-readers.
The ipad's screen size is perfect for reading pdfs. You can easily show your players what NPCs and areas look like via HD youtube videos (if you have wireless anyway, would not want to 3G that stuff). There are tons of good apps that make character sheets and power cards. It has a built in microphone so you can make an audio recording of your entire session. It also has built in speakers so you can quickly change the music to match the mood at the table. And unlike other e-readers, the iPad has apps that allow you to view flash content on the web.
iTouch/iPhone aren't bad, its just impossible to read pdfs on those tiny screens. Their biggest benefit is their super small size, but that doesn't doesn't matter much at a gaming table.
Both the Nook and Kindle are basically cheaper versions of the iPad. They do pdf/mp3/browsing just as good as the ipad. But they don't have video/speakers/mic. Both of them are lighter than the ipad (but not by much) and that's at the cost of some screen space.
If all you want is to read pdfs at the gaming table, then I'd recommend the Nook Color because its more affordable. The Kindle has some nasty DRM issues so I refuse to touch it. But if you're willing to fork out $400, you might as well go for an iPad1 just for the ability to view videos. In addition to using it at my gaming table, I also use it to watch netflix.
)
