how often does your cousin play Doom3? if she doesn't play a lot, you can consider buy onboard vga motherboard. either 865G, 915G are ok. 915G is about 2X faster than 865G in 3D. if she need faster graphic, she can alwasy get graphic card later.

you can also consider MATX motherboard.. which is smaller, and you can get smaller case (cuter)

make sure you get the P4 with HT enable. otherwise, it's useless.