The Alienware X51 R2 is a pretty unique desktop PC. It's small, compact yet very powerful. It should certainly make gamers happy. The downside of the compact design is that there isn't much room for upgrading or customising, but its small footprint does open up more options for placing it.
Dell lets you customise the system to a large extent. The configuration we tested isn't available anymore, and it's really too bad you currently are not able to select both a hard disk and an SSD because that is the ideal storage solution for a desktop. On the plus side, they did replace the mediocre WiFi card by a more powerful 802.11ac adapter.
The external power supply is also on the large side, it reduces the portability of the system somewhat. We can imagine gamers bringing this to a LAN party instead of a pricy gaming laptop that likely won't be as fast either.
Thee Alienware X51 with Core i3 and GT 645 is available from £599 / €699. The version with Core i7-4770 and GTX 760 has a starting price of £999 / €1,099.