Dell XPS X15Z-7500SLV or Lenovo IdeaPad Y580 or any other laptop?

The Lenovo Y580 is much better than the Dell for gaming. You can look up gaming performance of graphics chips at this site and you will see that the GTX 660M is far better than GT 525M or even GT 540M or 640M. You do not even get close until some competing GT 650M and still there's a notable difference.
http://www.notebookcheck.net/...849.0.html
CPU performance can be seen here:
http://www.cpubenchmark.net/...u_list.php
and this is a very fair review of the Y580 and is very informative:
http://www.notebookcheck.net/...974.0.html

Thin notebooks can't get the heat out, so they have to use lower performance CPUs and graphics chips and/or get extremely expensive.

You can also look at Lenovo.com for their on-sale Y580s and their delivery date and features and recommended to get the 1080p screen. The Y580 is a standard thickness laptop at 6.15 pounds. If you want to reduce the weight, you can get a Y480 with GT 650M in a smaller screen at x768 resolution (720p) that will be lighter and a bit thinner. The features of the Y580 are excellent with improved wireless a polymer battery, backlit keyboard, the high level of DRAM, graphics, and CPU. The only issue is the glare screen that reflects lighting behind you but that same screen makes a truer better color set.
Here is a detailed size and weight review of Y580:

There are competing models and also other Y580's at Newegg.
http://www.newegg.com/...6834152370
This MSI at $1150 is very similar.
This Y580 has the 1TB HDD without the SSD acceleration:
http://www.newegg.com/...6834310662
This MSI comes in a little cheaper with GT 650M instead of GTX 660M and no Bluray.
http://www.newegg.com/...6834152339
There are a few more at Newegg.
Check Lenovo.com directly for their offerings also.
With a GTX 660M you get high settings on Battlefield-3 even with the 1080p screen but not ultra.
The GT 525M of the Dell you found with a 1080p resolution will be slightly lagging at the lowest game settings.