I have the actual Linux OS on a separate partition. Then, my home directory, is on another partition.
I have my system fully set up. Ubuntu Hardy Heron 64 bit edition. Quite a number of programs installed... Frostwire, Audacious, Audacity, k3b, google earth, thunderbird, vlc, k3b, etc... And my OS partition is 15gb. I have 10gb free.
So, based on that, you can judge accordingly to how much space you will need when you add in the number of 'gb' your pictures and music will take up.
Also - Linux can read NTFS, however, it cannot write to it. I've been told with an additional plugin, installed via 'synaptic package manager', can be installed to grant write access to Linux so you can write to NTFS partitions. You may want to look into that... it sound possible to have all of your music and pictures on XP, however, Linux be able to read/write to that partition.
But I don't have much experience with that. Linux has always been my primary OS. XP's only place on my system is for gaming.