Hey cyd, I can hopefully help you out here. I'm also looking to switch to AT&T soon and have been researching almost every smarthphone they're offering, since I'm very picky when it comes to phones. Here's what I can tell you from what I've read and experienced. (Can you also tell me what phone and carrier you're with now?)
The most popular AT&T smartphones/PDAs right now are the HTC Fuze and the Blackberry Bold 9000. Although they're more expensive than the rest of the AT&T phones, they offer the best features which really give the biggest bang for your buck.
Blackberry Bold 9000 Although I haven't spent much time with this phone, I've heard nothing but great reviews about it, particularly from MTR's very own review which can be found here. http://www.mobiletechreview.com/phones/BlackBerry-Bold.htm Although its not a touchscreen like you prefer, it still has some great features that may convince you to consider the phone. The design looks and feels great, the screen has the sharpest displays on the market, the new Bold interface is VERY fast (applications open up immediately with no lag), and it has all the smartphone features such as a great web browser that features full HTML browsing, WiFi/3G/GPS capabilities, and great multimedia performance. Some negatives about the phone could be the large size (depending on what phone you're coming from), and I've heard some Bold users complain about the short battery life. But then that would depend on your usage. I can't comment on Blackberry's famous email capabilities since I haven't had experience with it.
HTC Fuze If you prefer a touchscreen phone with the Windows Mobile interface, this would probably be the best phone. Again, it's pricier than the competition, but some features help make it worth the price. First off, this has a full QWERTY keyboard that slides out and doesn't sacrifice the large screen size. It has 3G/WiFi/GPS capabilities like a true smartphone, full HTML browsing experience and even includes the Opera Mobile browser (which, in my experience, is a much better faster and better browser than the Pocket IE). The TouchFlo 3D phone interface is true eye candy. I'm pretty sure you're familiar with it if you've read some reviews (http://www.mobiletechreview.com/phones/HTC-Fuze.htm). The downside of the phone, according to some users, is its performance lag. Although the phone has a fast processor and lots of RAM, all the AT&T software loaded on the phone seems to affect its performance. Users complain that the device occasionally lags and (in some cases) freezes. BUT the best part about a Windows Mobile OS is that you can tweak and customize the phone to get rid of the bloated software and help increase its performance. There are also tons of 3rd party software to help customize your phone to your liking.
IF you're on a budget, there's still hope in finding a great smartphone. AT&T also has the Samsung Epix, LG Incite, and the Palm Centro.
Since I don't know ANYTHING about the 3 phones listed above, I'll provide links to videos and reviews that can help you decide on the phone that best fits your needs.