Well the the two current big boys are the Audiovox SMT5600 (which has about 20 other model names and is really made by HTC) and the Motorola MPX220. There are also some older Samsung i600 and Moto MPX200 and various others on the market, but the 5600 and 220 are the latest and greatest.
I don't know about the 220 but the 5600 has a 200MHz processor, 32 MB of Ram, and 64 of built in storage... both the 220 and 5600 have MiniSD slots and both currently run anything up to 512... it's unknown whether they can handle the new 1GB miniSD cards which aren't widely available yet.
You can look at
http://www.Microsoft.com/smartphone for specs on the major phones. There are also the Palm phones, etc, but I would suspect that porting to the MS Smartphone would be easier since it's supposed to be a derivative of the Pocket PC.