prebuilt.... if you can find a laptop with an amd a10 processor in it, or one with in intel i3 and either an amd hd7730m or a gt630m or higher you will be fine.
though your local bestbuy probably has a desktop with an amd a8 processor or higher in it and it would do the job too.
http://www.bestbuy.com/site/essentio-desktop-8gb-memory...
that desktop has a quad core a10 with built in hd8670d graphics which capable of playing many modern games at medium to high settings. only in a few handfuls of ultra leading edge graphical games will it start to struggle.
http://www.notebookcheck.net/AMD-Radeon-HD-8670D.96575....
for a scale of what its capable, according to above benchmarks, its capable of playing call of duty black ops 2 at 57fps running medium settings at 720p(1366x768), which would make it considerably faster than a ps3 or xbox360.
i have a laptop with a slightly different amd a10 processor with similar graphics that plays battlefied 3 or skyrim @ 720p medium settings no problem, though my laptop is at least 33% slower than the desktop version since its designed to run on very low power.