No laptops are not like desktops. It is much easier to upgrade a desktop's video card. It's either impossible or much harder to upgrade a laptop. Pretty sure the latter would require soldering.
If you want cheap, go for a desktop. If you want portability, you can get a decent laptop for around 600-700. I bough an HP DV6 gaming laptop with a radeon 4650. So far I've had no problems running games. Borderlands, MW2, Arma 2, L4D2, etc all run smooth.
What I would suggest is figure out if you want something you can upgrade later and would cost much less.(Desktop)
Or
If you want portability at a steeper price without the ability to upgrade later on.
Also try checking out sites like newegg. That's where I purchased mine at a pretty reasonable price.