Hi Richard,
with my 2-Xeon-CPU-Box I experience the same behaviour.
Almost each and every scene, especially the population heavier ones are sluggish in the viewport.
This all started with the introduction of the RTP.
When I try to move the viewport there is a more or less longer pause (like "thinking thinking thinking a that`s what he wants!) before the viewport reacts. I usually have only bounding box display on to be able to do anything in time. With imported objects and populations that is.
Rendering times - I could not be happier (only if I was a rich man...dabadabidibidu)
Then I do not really take those benchmarks too seriously. I ran it about 10 times now, because the first time it broke down, couldn`t finish, after that quite a variety of results each time. Ranging from very WOW to way below expectations. So that either tells me my system is completely f#*#d up or the benchmark is soso...
I do not think it`s the RAM or Graphics Card on your system. TG does not utilize the most modern features of GPUs, I think I remember Matt or Oshyan say the viewport uses good old OpenGL mostly or even exclusively. Every G-card should be able to handle that nowadays.
Why would your older system with a assumably older Gcard work more smoothly? You even have only 1/2 the amount of RAM in your old box.
The main difference between your old and your new system seems to be the CPU and the amount of cores.
You did not say which OS you are running, though.
My main advice would be to try different (even older) drivers for the Graphics Card and double check for all your drivers and utilities of the mainboard. They truly make a huge difference. Chipset drivers especially.
And regarding dust...well I think a system that is watercooled has other issues than dust.
You should monitor the temperature of your cpu over a longer time period to see if the watercooling works as expected - on the other hand if the box renders as fast as expected and does not show any signs of "getting too hot too easily" I would think that this is not the cause to sluggishness.
So, what does this mean - nothing at all. Just a guess as good as anyone elses, really.
Good luck! CHeers, Klaus