I've not seen much on overclocking graphics cards and CPU to get a combined effect so I gave it a go. I don't know if anyone will be interested either but I spent all night getting a fast stable system and thought I wack up the fruits of my labour for all to see.


Here are the results:

System:
Motherboard: Abit IP35 (Socket 775) PCI-Express DDR2
Processor: Intel Core 2 Quad LGA775 Q6600 2.40GHz
Fan: Zalman CNPS9700-LED CPU Cooler
Memory: Corsair 2GB DDR2 XMS2-6400C4 TwinX (4x1GB)
Video Card # 1: BFG GeForce 8800GT OC 2 512MB GDDR3
Hard Drive # 1: Western Digital Raptor X 150GB 10000RPM SATA
Hard Drive # 2: Western Digital RE2 500GB SATA-II 16MB Cache
Power Supply: Corsair HX 620W ATX2.2 Modular SLI Compliant PSU (CMPSU-620HXUK)
Monitor: L2442W 24" Widescreen LCD

Here is the stock setup:

CPU = 2.4 (1.28 Volts)
GPU = core bus 675 membus 975 (yes I know already a bit clocked by factory)

Temps:
idle Prime Crysis
CPU 33 46 38
Syst 34 34 35
PWM 39 49 39
GPU 50 50 75


Max stable clock

CPU = 3.5 (1.39Volts)
CPU = core bus 743 mem bus 1014

Temps:
idle Prime Crysis
CPU 35 60 42
Syst 34 34 35
PWM 39 71 49
GPU 50 50 75

And if your interested here are the details of 3DMark06:

All Stock:
Main Test Results
3DMark Score 11853 3DMarks
SM 2.0 Score 4886 Marks
SM 3.0 Score 5161 Marks
CPU Score 3596 Marks
Detailed Test Results
Graphics Tests
1 - Return to Proxycon 40.219 FPS
2 - Firefly Forest 41.216 FPS
CPU Tests
CPU1 - Red Valley 1.186 FPS
CPU2 - Red Valley 1.744 FPS
HDR Tests
1 - Canyon Flight (SM 3.0) 51.585 FPS
2 - Deep Freeze (SM 3.0) 51.632 FPS

Only CPU overclocked ( Gfx car stock)
Main Test Results
3DMark Score 13677 3DMarks
SM 2.0 Score 5760 Marks
SM 3.0 Score 5391 Marks
CPU Score 4946 Marks
Detailed Test Results
Graphics Tests
1 - Return to Proxycon 47.158 FPS
2 - Firefly Forest 48.845 FPS
CPU Tests
CPU1 - Red Valley 1.635 FPS
CPU2 - Red Valley 2.393 FPS
HDR Tests
1 - Canyon Flight (SM 3.0) 51.583 FPS
2 - Deep Freeze (SM 3.0) 56.231 FPS

CPU and Gfx card overclocked:
Main Test Results
3DMark Score 14473 3DMarks
SM 2.0 Score 6171 Marks
SM 3.0 Score 5754 Marks
CPU Score 4972 Marks
Detailed Test Results
Graphics Tests
1 - Return to Proxycon 50.723 FPS
2 - Firefly Forest 52.119 FPS
CPU Tests
CPU1 - Red Valley 1.657 FPS
CPU2 - Red Valley 2.387 FPS
HDR Tests
1 - Canyon Flight (SM 3.0) 55.594 FPS
2 - Deep Freeze (SM 3.0) 59.482 FPS

conclusion
In conclusion I think you can get a reasonable benefit to gaming from overclocking but it's not as huge as one might hope.

I did find adding the fan from the Intel CPU cooler to the top of the G8800GT made a difference to the GPU cooling quite a bit, without it the temps went way up even running stock.

I have to say I'm not happy running 24/7 with the PWM getting that hot all the time so I may add some 40mm fans to help them out.