The rz1715 - Their low-end Mobile Media Companion, the rz1715 only has a 203MHz processor, 32MB of RAM, a 3.5-inch LCDscreen, an SD expansion card slot, but no WiFi or Bluetooth.
The rx3100 - They're dubbing the rx3100 a "Mobile Media Companion", and it comes with 64MB of RAM, built-in 802.11b WiFi and Bluetooth, a 3.5-inch LCDscreen, an SD expansion card slot, and software for wirelessly browsing your music, photo, and video collection from anywhere in your home and so you can use the rx3100 as a universal remote control.
The rx3700 - Another Mobile Media Companion, the rx3700 has the same media and universal remote software as the rx3100, a 400MHz processbuilt-in WiFi and Bluetooth, a 3.5-inch LCDscreen, and an SD expansion card slot, but also has a built-in 1.2 megapixel digital camera.