1. As Peace said, current PowerMacs have 2 CPUs. Each CPU is single core, thus 2 cores total.
2. The common interpretation here is that if a quad Mac comes to exist, it will have 2 dual-core CPUs, for a total of 4 cores.
3. Hyperthreading is a technique used by some Intel CPUs to make a single core look like 2 logical processors. For some tasks, this improves performance by letting certain operations execute in parallel while others are stuck waiting for something. For the most part, the benefit is fairly small, and not nearly enough as if you have two physical cores. You still only have one core, so it can only do so much at once.
So in theory it would be possible to have 2 dual-core CPUs with hyperthreading. This would give a total of 8 logical CPUs, made up of 4 physical cores on 2 physical CPUs. As far as I know, IBM has not done anything with hyperthreading, but they do have a new dual-core CPU. It's hoped that new G5s will use these. Whether they'll stay with 2 physical CPUs (giving us 4 cores, twice as many as before), or whether they'll use this to drop down to 1 physical CPU with 2 cores (therefore the same performance in less space), we don't know for sure.
