10 should be more than enough for XP, i am running 2ksp3 on a 3gb partition.
i am currently still working on my mini-itx setup. i also have a great challenge because i cannot have moving parts in it (no fans, no hdds). i bought a 1gb compact flash card and an IDE adaptor for it and i am running knoppix linux on it which is customized to fit my needs.
i was wondering, where did you get your PSU and how much was it? i am in need of one because my micro-atx psu died.