Why not ask your gf pick her favourite casing first? I think the appearance, size, color and casing design matter the most to ladies. They mostly dun care if it's Intel or AMD inside, or DDR2, DDR3 or even GDDR5 for memory, so long as it runs Windows.
Then you see whether it's a mini-itx or m-atx board, and whether it accepts ATX PSU or some other funky SFX or TFX PSU?