Norton check's speed of CPU's? I would ignore Norton, but you may want to check a real benchmark to make sure it is working properly. The i7 5820K is one of the fastest CPU's out there, especially when OC'ed past 4Ghz (the stock frequency is low).
I'm not really an expert on OCing. I do have mine at 4.375Ghz (I also just bought one a month ago). I can hit 4.5Ghz, but prefer to back down for some wiggle room. Make sure to have a good quality CPU cooling solution. Having 6 real cores adds to the heat.
As far as benching goes, that depends on what you plan to use this CPU for. You can look at a review site for the i7 5820K, and use what benchmarks they do.
I'm not sure on the best price to performance ratio AMD 8Gb cards, but the r9 390 and r9 390x are the only ones that normally come at 8Gb. I'm not sure why you need 8Gb though.