Something that has a decent amount of RAM (Hmmm say, 8GB or more, you can never have too much, but RAM is also the absolute easiest thing to upgrade), and a decent CPU (>2.5GHz quad core or better, accept no substitutes). Off-the-shelf systems often do not include a decent video card, but if you're lucky you'll get one. More than likely, you will find a computer with a good CPU, good amount of RAM, but a sucky video card which you will have to replace.
You will have to research graphics cards as it's difficult to tell by the name. This is a very nifty site for comparisons [http://www.notebookcheck.net/Mobile-Graphics-Cards-Benchmark-List.844.0.html], and so is this one [http://www.videocardbenchmark.net/high_end_gpus.html].
There you go.