Dig Deep into your own Performance
While OSX has a handy tool called "Activity Monitor" that can show you how hard your CPU cores and HDD's are working this is just the high-level information. If you *really* want to see inside your hardware's performance from an electrical surgeons point of view and actually see exactly how your CPU, GPU and RAM are crunching data - and how efficiently - you have super-cool tools readily available to you built right into Mac OSX.
Get out your Mac OS install disk and in the Optional Installs folder install the Developer Tools. This will install a host of code-monkey and other diagnostic tools that can do a lot more than just provide performance benchmarking, there are things that can show software hangs, "busted" code, control the spin-down of HDD's... it's just amazing what's available. Windows users have never had tools this powerful - unless you pay hundreds of dollars for a Windows SDK.
Specifically the 2 tools you can call up to dig into your GPU performance are the "OpenGL Driver Monitor"; this shows a plethora of stats you can apply to the performance window and watch in real-time as you work, how the GPU is being used by any application - especially Compressor or FCP during renders. This is a developers tool so there's no simple bar-graph like in Activity Monitor but you can choose as many parameters as you like and watch your video card at work.
The second is a multi-tool called, "Instruments". This one has many sub-diagnostic tools which amongst other things can display the performance of your multi-core CPU, threads and CPU performance.
The Developers toolkit does come with a PDF which explains each tool and how to use it, so read over that stuff before you go digging around but these 2 tools can very easily show you how well - or not - your system is crunching renders and encodes on ANY application, not just FCP.