Tip: bc(1) (bundled in most Unix-like systems, including OS X) is handy for converting between bases.

ringo:~ tim$ bc
bc 1.05
Copyright 1991, 1992, 1993, 1994, 1997, 1998 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 
ibase=10
obase=16
255
FF
1024
400
1239087123
49DAF813