If replacing the 3820 with a 6700k, you will gain raw performance, but will lose out on available PCI-E lanes for 3-way SLI, PCI-E SSDs, etc. The 6700k uses up to 16 PCI-E lanes (plus four dedicated to storage) and the 3820 uses up to 40 PCI-E lanes. If you need all of those PCI-E lanes for...