Its doable to upgrade the laptop GPU, but its difficult, you'd probably void your warranty, and the new GPU cards are often hard to find.
For a notebook: I'd suggest looking at this: http://www.newegg.com/Product/Product.aspx?Item=N82E16834230099
which is the cheapest I've seen with a decent GPU. Its not going to be the best, but Damned good for a laptop.
Either that or take a look at the MSI range.
Don't even bother with HP or Toshiba (etc) as they are more focused on business machines!