I know that if I use Pooler's CPUMiner it will use all my computer resources for mining purposes.
What if I use one of those Butterfly Labs 5 GH/s SC devices will it use 100% of my CPU power or the device does all the hard work?
If you are using dedicated mining hardware, then CPU is only used for dispatching work to ASIC and sending the result back to pool. Therefore, CPU usage in this case should be very small, certainly not 100%.
Own experience: Linux, cgminer, 2*5GH/s ASICs: the miner uses about 1 minute of CPU time per 24 hours walltime, much less then bitcoin client running mostly in background.
A picture is worth a thousand words. Here is a screen capture of a Mac Mini rig with three BFL USB devices running just under 70 GH/s. The low-end Mac is not even breaking a sweat... You probably need to figure out what processes are sucking up your CPU resources and kill them. I highly recommend http://fabulouspanda.co.uk/macminer/ and doing STM P2Pool mining at http://pool.fabulouspanda.co.uk:9332.