The 2600K is still a pretty decent CPU (especially if yours is OC'd) and the only reason I upgraded mine to a 4930K was because I run some ridiculous video settings and my CPU was becoming a bottleneck (I also added a third GPU). I actually kept my 2600K system and use it as my secondary computer (replaced an old Q9550).

If you want today's equivalent of a 2600K CPU then you're looking at a 4770K, but I don't think it warrants an upgrade unless you are sure that the CPU is your bottleneck. If you really want a new 6-core upgrade, then the 4930K is it, but next year Intel is releasing their X99 chipset and will be selling 8-core CPUs at the high-end consumer level -- We're still probably about a year away from that happening, though.

Also, more memory isn't going to do anything for you unless you're running out of it, and I doubt you are since you say you're only running 3 clients (I realize you've already ordered it). In fact, the more DIMM slots you fill up with memory, the less stable an overclock can become (or it may be harder to hit certain speeds) -- Again, only important if you actually OC your system.

Whether you use ISBoxer or not, this page has a good way to identify any bottlenecks in your system:
http://isboxer.com/wiki/HOWTO:Tweak_...te_bottlenecks