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Like Yosemite and Mavericks before it, El Capitan can run on the following Macs:
iMac (Mid-2007 or later)
MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)
MacBook Pro (13-inch, Mid-2009 or later), (15-inch, Mid/Late 2007 or later), (17-inch, Late 2007 or later)
MacBook Air (Late 2008 or later)
Mac Mini (Early 2009 or later)
Mac Pro (Early 2008 or later)
Xserve (Early 2009)
All of these Macs feature a 64-bit CPU.
Apple hasn't changed the requirements for OS X in a number of years, many of these Macs are now seven to eight years old.
So you can update to El Capitan, but Quark 8.15 may have problems on running it. Quark recommends QuarkXPress 2015 for El Capitan.