For starters, I thought that I would mention the easier alternative to the "hard-mod", which would be a BIOS flash. Just flash a standard GS or GSO with the BIOS from a GS/SC edition, and you will be able to enjoy 650mhz startup core speeds as well as increased shader clock of 1620mhz
For starters; the Vmod candidate. A standard PCB rev, GS/GSO/GT/GTS all have this similar layout..and this mod works for all of them.
Make sure that your GS/GSO is a standard PCB revision type [as many do exist, XFX likes re-arrange PCB and cap layouts..and can make things a little difficult sometimes]..this guide will focus on standard PCB rev, only.
what you need:
1. 500 Ohm Variable Resistor
2. DMM [Digital MultiMeter] both for voltage measurement and tuning the VR's before installation.
3. 12-18W solder gun
4. Solder [of course the smaller diameter and least amount of lead FTW.
5. Electrical tape
6. Super Glue
7. An extra IDE cable
8. Small flat-tip screw driver for adjusting the VR
note:8800GS/SC in this test
Preparation:
First, you will want to tape off the mod area, as carefully as you can, using the electrical tape. Cut short 1-2" pieces and place them around the resistor to prevent damaging other components on the PCB. Making sure to leave a small opening in the center, for us to solder our VR in-line.
Completed:
Now that that is done, lets have a look at the VR's. Remove one of the outer legs on the VR [either one is fine], leaving only one outer leg and the center leg, still attached. Using your DMM, set to Ohms, and clip the ground lead to your VR, the posistive lead to the outer leg on the VR. We are now reading resistance through the VR..and need to set it for its max value of ~500 Ohms. You must do this step NOW, before connecting the VR to the GPU..as you will not be able to read the resistance level, once the VR is soldered into the circuit. Since the resistance level directly corresponds to the new voltage level..you risk blowing your GPU if you do not tune the VR before soldering it in-line.
VR, *out of box*-
VR, ready for installation-
Measuring resistance on the VR
*** A very important detail-> Make sure you have identified which way increases resistance for your VR. This is a crucial part..mark your VR, with a pen to indicate which direction you should turn to raise Resistance [which will lower GPUv]
Wiring:
I like to start with an old IDE ribbon cable. By cutting a few of the wires, at the back of the connectors, you can strip off a few together.
IDE cable..cut-
Solder !
I prefer to solder my wires up to the VR first, as the connections to the IC chip are very fragile and can break off easily if moved too much. After you have soldered your connections to the VR, use some of the super glue to mount the VR an exposed edge of the GPU's PCB.
Connections to VR complete
VR mounted
-
Now to solder the connection from the middle leg of the VR [Ground] to one of the many ground positions on the PCB. I chose a ground point close to the GPUv measure point.
Soldered Ground connection
Making the next connection takes patience and some skill...practice before hand, you will want to get this right within the first 2 attempts, are it is likely, you will burn the 'pad' and not be able to mod after that. Since we are not landing directly to an IC or a resistor, there is little risk in damaging the card from burning this solder pad...there is a risk you can burn or solder a surrounding component..so tape off the area WELL before proceeding.
Finished result, after removing tape
Double-Check :
With you DMM, set to continuity [*beeps* when you touch the [+] to the [-]
] Check for the *beep, by measuring across both legs of the VR..if you have tone, your VR is hooked up and in-line... if not, you need to re-solder one of your connections. 
After you have verified the connections are good, you 'seal' them up with a small dot of superglue, to make sure they dont break off, moving the GPU around.Assuming all is well..Lets move on...
1. Secure any loose wires with some more super-glue, or a piece of electrical tape. Then re-insert your GPU, back into the mobo..DO NOT TURN ON YET!!!
2. Set your DMM to Volts DC and get ready to measure GPUv as we start up.
*Find bare metal in your case, for the [-] lead, and place [+] lead on one of the GPUv measure points in the first photo.
3. Fire up the rig, as you monitor GPUv..if you see volts in excess of ~1.28-1.3v..shut down and increase the resistance on your VR. *Using the 500 Ohm VR set at ~max, you should have a slight increase in GPUv over stock..this will prevent not having enough voltage to start the GPU upon Boot-up.
4. With a stock GPUv of ~1.08v under load, I decided to start with 1.18v ..and see how the GPU responded to core increases and to monitor temps.
Testing & Verification
After monitoring my temps and a few back-to-back 3D runs of 06' and 05'..I ended up with the following results:
*-Stock 8800GS/SC: 650mhz core / 1620mhz shader / 950mhz vmem
1.08v under load, ~1.06v idle
Vmod'd 8800GS/SC: 804mhz core / 1970mhz shader / 950mhz vmem
1.25v under load, ~1.21v idle:
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