If you get a laptop and a desktop of equal power, the laptop would be far more expensive. You should definitely build your own desktop, pre-built PC's are usually very poor for gaming and have unbalanced components, if you do find a decent one, you will pay through the nose for it, especially if you buy from Dell.
Building your own PC is fairly easy, you can tailor it to your needs and you will save a lot of money. If you're only going to be doing some light gaming, then I recommend a low end i5 quad core and a mid-range graphics card like the Nvidia 550ti or ATI 6770, which is very similar to the build I'm working on at the moment. You can build the rest of the system around that. It should cost you around £400, including an OEM copy of Windows 7, you can always downgrade to an i3 and lower end graphics card to cut your budget down to around £325.
