hi,
this is a HPC forum, not a PPC forum, but you're right about the confusion around this... CE 4.20 is the core OS of your platform, WM2003/2003SE.
so... don't worry, you have WM2003 and can buy the card.
you can consider yourself really lucky, we HPC users always have this problem for real
(have the core OS but not the platform, leading to compatibility problems
).