Which file to use? 

First determine which type of processor do you have: ARM, MIPS, SH3
If you don't know, it's most likely ARM.

*.ARM.CAB 
  PocketPC, PocketPC Phone Edition, HPC and other ARM handheld devices
  (Intel XScale is an ARM cpu)

*.ARM_SP.CAB
  MS Smartphones (for SP2002,WM2003 SP,WM2003SE SP)

*.ARM720.CAB
  Same as ARM, just with different ARM CPU identifier for Sigmarion III
  (this is NOT for Pocket Loox 720, please use TCPMP.ARM.CAB for that device)

*.MIPS.CAB
  Devices with MIPS CPU

*.SH3.CAB
  Devices with SH3 CPU