I assume your mum isn't a gamer

and this pc will be used for general email checking, sending embarrissing photos to realitives and thats about it?
So A celeron/sempron would be better suited, as it is cheaper, and will do above fine.
1gb of ram is overkill forn running a OS and IE and outlook.
I doubt she would use up a 250gb hdd unless she will be recording TV on it (explains media edition of XP.)
No power supply? You will need a 24pin one.