Download SpeedFan, CPUz & Orthos.
As the computer boots hit the delete key - you will enter the BIOS.
Select Advanced Chipset features - DRAM Configuration. Switch Timing Mode from Auto to Manual. Hit Enter. Select Memclock Index Value. Switch from 200MHz to 166MHz. Hit Enter. 'Escape' back to main BIOS screen.
Select Cell Menu. I think (can't remember

) you select Performance mode. Switch setting from Auto to Manual. Hit enter. Disable Cool & Quiet. Verify that your PCIe freq is set at 100MHz. Select HT Freq. Switch setting from 5x to 4x. Hit Enter. NOW ....
You have room with these settings for an overclock up to 25% above stock. I would suggest starting slowly, working you way up and monitoring your system along the way.
Select CPU FSB Freq. Switch the setting from 200 to 220. Hit Enter. Hit F-10 and save the new system settings. After you boot into Windows open CPUz & SpeedFan. Your clockspeed should be 2200MHz (or a 10% OC). Note the CPU & Core temps in SpeedFan. Write them down!
Right click on the task bar and open the task manager. Select the performance tab. Launch Orthos and select the cpu/ram 'blend' test. Hit Start.
Your CPU utilization should rise to 100% on both cores in task manager. Monitor your CPU & Core temps in SpeedFan. At this OC most likely your load temps won't rise above 40c. If they do, you may need to reseat your HSF. NOW .... It's up to you!
If you are comfortable with your stability and temps, with the settings I gave you you could up the CPU FSB Freq as high as 250 (that would be 2.5GHz or a 25% OC). If along the way the system becomes unstable or won't boot, you can go in the Cell Menu and safely raise your CPU Voltage to 1.425v - anything above that and you are on your own

If you can't enter the BIOS read the manual on how to reset your CMOS. I don't know anything about your ram so I will not suggest raising the mem voltage.
If you are happy with your system stability and temps and wish to proceed higher that 250 CPU fsb - you must reduce your HT from 4x to 3x and your MemClock from 166 to 150. Some folks have clocked X2 3800s to 2.7GHz and up. You can most likey safely overclock your X23800 to 48-50c temps @ load in Orthos.
I'll let someone else explain how to 'relax' your memory timings. If you run into problems RESET your BIOS to standard defaults (and read in your manual how to reset the CMOS!)
And while at lower resolutions you may be slowed a bit with your cpu - at 1400x900 you are starting to enter territory where your 6800gs may start slowing you down. If you are so interested heck out something like the
8800gs for a big bump in your video card ...