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Can I run the cool 320x480 apps on 320x320? If not, maybe get the TX.

Most apps run natively in 320x320, and some do not support the big screen. It is harder to find apps that only run in 320x480, but there might be a few that exist.

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Can I learn to scribble on the TX? If not, maybe get Treo or C.

You should be able to, but don't expect it to be faster than a keyboard. You can always get an extra wireless keyboard, and in my experience (I've owned several Palms, including a Tungsten C) I don't text input much, just read a lot and sync the verbose stuff with my PC.

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Are the Treo and C good enough for "1st person GPS maps" with audio directions?

Ask Mark Lord, I think he uses a maps/GPS program with his Zire. I know the TomTom software for the Palm is pretty slick, but never used it myself.

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Can I get a Wireless SD/MMC card to network the Treo?

No

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Does anything make the C better than the other two?

Wifi + Keyboard (TX only has Wifi)

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Can I get a big SD/MMC card and use the Palm for file storage?

With additional software, they can act like a removable USB drive. You can of course pop the SD card out and use it for file storage, too.

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What's this about the Treo and C need an expansion card to play MP3s??

The Treo and C needs to have MP3s stored on a SD card, you can't store them in memory (or I've never done it). The T5 has a special memory area that acts like an expansion card and you can store MP3s internally for playback. The Tungsten C is no good for listening to music because the audio jack is mono only. MONO ONLY. I heard that they were going to port some kind of Skype-like phone software to it and that is why they have the mono headset jack instead of the stereo jack.

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Is the trusty T5 simply dwarfed by the TX?

Not entirely. The T5 still has drive mode, where you can use it natively (wthout extra software) as a USB drive. It also comes with 256M of memory vs. 128 in the TX, and has a faster processor. The battery life is a bit longer, too. I considered getting a TX, but I really didn't use the wifi part of my C much when I owned it, and I like the faster processor for watching movies with TCPMP. That is a must have for any T3/T5/TX/Lifedrive owner. Get a 1gig SD card, rip some DVDs using FairUse Wizard and I can fit 3-4 movies on one SD card and watch two of them on one charge. Great for trips on the plane. I also use my Palm as my portable MP3 player.
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Mark Cushman