Ubuntu won't boot from inside a RAID volume anyway - and onboard RAID is often Windows Only.
Full HW RAID should be fine. Note RAID1 is not a backup strategy.
So - your decision to move away from RAID is probably a good idea.
Free software advocates will tell you you do not want or need XP.
If you do not install XP, then you can use linux software raid on your two big drives
How abouts putting XP on a small drive while you get used to not using it?
However - your options are wide open ... linux does not insist on having a drive to itself. Windows does not play well with the other kiddies... and it doesn't like linux file systems.
Putting Windows and XP on different drives is a good idea - as you are new to all this, keep it simple. I'd run with just the small drives until I got used to it, then make a final decision about what configuration I want. (Whatever you pick first is seldom what you end up wanting.)
Install XP first (if you must) then install Ubuntu. Put XP on the first drive. Ubuntu will detect it and provide options - accept the defaults.
Couldn't be simpler.