Now CPU-wise ok I might be $120 over from getting cheapo single cores. When I bought an X3 and saw that the two cards pegged it to 100%, I wasn't sure if it was just a bug compared to the CPU being powerful enough to keep the cards fed. I think now it is rather the former and if a single core is really good enough to mine with then that's good to know.
CPU usage bug depends from cards, drivers and system you're using, if you are using linux you will probably be able to avoid this bug but if windows is a must then in the task manager you can change cpu affinity for your miner and make it run only on one core so CPU usage falls to 50% (assuming no other cpu resource consuming apps are not running in the background).
On 12.1 drivers with 2.6 sdk I noticed lower hashrates when miner was using 100% of core0 on Q9300 @ 1.8GHz. Only when I let it use all 4 cores would it return back to 'normal' speed.
No such issue on previous drivers (11.5, 2.3 sdk)