It all depends:
What are you doing with your QX6800 rig? If you are web surfing, running Office and playing Facebook games it will serve you well for several years to come. Hell, I am using a Pentium 4 laptop at work to run office, adobe acrobat and do web surfing. It works just fine.
If you want to play games with your QX6800 (like WoW or SWTOR), according to PASS MARK http://www.cpubenchmark.net/overclocked_cpus.html
a modestly OC'd QX6800 (3.55GHz) performs the same with an overclocked Core i5-760, so I am guessing if you OC it to 4 GHz it will perform even better.
If you want to play high end games at resolutions over 1920X1080 and ULTRA details I guess that maybe you should upgrade to a Haswell platform.
Core i7-860 gets 6300 CPU marks while your overclocked QX6800 could probably do around 5.2k CPU marks at 4GHz. The difference is not worth it. A new Haswell would be a much more worthy upgrade value-for-money wise and significantly more powerful than your quite old QX6800.Edited by Arxontas - 6/20/13 at 11:38am