There's a trick to getting the optimum balance of CPU and memory use. Watch for disk activity that migt indicate excessive paging. On my iMac, which has a Core 2 Duo and 3GB of usable memory, I use only 2 processes and a bucket size of 32 (or 24 for a complicated scene). On a heavy render the machine's a bit sluggish, but usable for simple tasks. I find there's no point in trying to do anything memory-hungry or time-critical while a render's running...