I haven't been following the GPU business for a while, since my 4850 does what I ask of it, so I'm a bit out of date when it comes to what models are good for the money.
Anyway, my girlfriend is looking to do some upgrades to her prebuilt HP (an A6700Y), and looking at the specs of it, I'm not seeing much that really needs to be upgraded other than putting in a video card, and ditching the factory PSU.
Anyhow, for the PSU, I'm thinking something along the lines of a 400CX, but I'm not sure on a video card. She's looking to do the upgrades later in the Fall or over winter break (we're both students), and the budget isn't large, but considering that she's not a huge gamer, the card doesn't need to be either.
-Adam