I've always put systems together rather than buying one pre-built. If you're building one, I'd just say don't skimp on the video card- buy one with as much onboard RAM as possible, and get something like an NVIDIA card since the game companies tend to optimize their games for those. (For the 3D rendering and such.)

The mister just bought this one and is really enjoying it. He plays WoW and has the settings on "Ultra" and it's just flying. It took him about 2.5 hours to load WoW, the two expansion packs and the patches. It took me on my five year old pc about 15 hours to do the same thing. He says it's considered a mid-level machine for gaming.

Lucky me, he feels guilty about having a better pc and we're gonna get me one next week, yay!

I'd just build one. Especially if she wants a game-spec machine without a ridiculous case covered in blinkenlights.