As far as prebuilts go, Dell really has the best price-to-performance ratio. If you can't build one yourself or have on built, Dell is the way to go.
If you build it yourself, you will understand it far better than jst buying one, and so upgrading it will also be easier because YOU know what you put in it.
Besidees, IIRC, Dell locks their CPU/GPU frequencies, meaning you can't overclock.