I've been overclocking both the CPU and the NVidia 9400M graphics card on my MacBook Pro 13" 2.53 GHz ever since I took it home without any problems. Using the NVidia-provided System Tools Performance Feature
http://www.nvidia.com/object/nvidia_system_tools_6.06.html
you can set up profiles which can be used at Windows start-up or when launching a game. In Windows XP you even have the option to alllow the program to recommend an optimal overclocking frequency, which for my 9400M was quite high (563 MHz core graphics clock). Unfortunately, this option is unavailable in Windows 7 64-bit mode, but I currently have my GPU clocked to the same frequencies as the 330M (575/1265) with the CPU running at 300 MHz front side bus speed.
I recommend you download the excellent Fan Controller from Lubbo, which can be found at
http://sourceforge.net/projects/lubbofancontrol/ and which is far superior to the default fan settings. My GPU temperatures never exceed 80° C with this utility.
Of course, overclocking is not recommended and you need to be aware of the dangers involved, including violation of your warranty. But my experience has been that so long as temperatures are under control then go for it!
I haven't found a way to overclock in Mac OS X, but since I use the Boot Camp partition for gaming and want the most out of the GPU this hasn't been an issue for me.
Here's hoping the new 13" MBPs will feature the NVidia 330M with 1 GB dedicated RAM! They're reported as being 4x faster than the current 9400M variant, so I just might give my laptop to my wife and go for the new model when it finally arrives. Maybe this coming Tuesday?