It’s a decent spec, note that it doesn’t come with a monitor.
For perspective, if you were to get the same spec from scan.co.uk as separate components and build it yourself, it comes to around ~£820 (this is rough and ready, using midrange components for anything not specified, eg motherboard).
You’ll probably be fine with an i5 (say, 3570k) instead of an i7. You’ll probably want a nice sound card (makes a massive difference to games) – the alienware doesn’t come with one.
Have a look at this:
Scan Premade System
Better graphics card, double the storage space, comes with a sound card, also comes with an SSD (very fast hard drive), for £50 more than the Alienware