Doing xp first is easier since xp will overwrite grub and make ubuntu unbootable if you install it after it.
Don't hide any partitions, just make 3 or 4 primaries and go from there
Don't use any extended partitions.
You'll probably want to partition it roughly like this -
/dev/sda1 ntfs 100 gig or more
/dev/sda2 ext3 /boot 512MB
/dev/sda3 ext4 / 100 gig or more or /dev/sda3 lvm
/dev/sda4 swap 2 gig
lvm's nice, fedora uses it but ubuntu doesn't by default
There's more info on fedora over here http://www.tomshardware.com/forum/236837-50-step-step-linux-installation-guide-2010-fedora-x86_64