First of all X-15 is a gaming notebook...not desktop.
the desktop from Alienware will roughly cost 1500 US$ plus. the x51 will cost like 1150 US $.

The other option is buying ACER M3800 / 5800 Machine or ACER Predator G3 Series Desktop (1050 US $) and install a high end video card of your choice which will probably cost another 450 US $ + LCD