I did this modification with great success on my SC1420
What I did was to remove these tiny two "blockers" (red circle) with a regular dremel multi tool.
Before you start, remover all the ram, cpus, fans, and power cord.
If you dont have antistatic gear, touch the frame where there is direct connection to metal
several times, several places.
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Remember: Cut VERY carefully when you approach the actual pins.
At first there are no pins (circled in blue) but at the second "blocker" there are
working pins for the PCIe card.
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I used a very sharp scalpel the very last way to carve off the remains.
Be very careful about the two transistors that are on the side of the pcie 8x slot, and
not to cut in the pins inside the slot.
I then put my ASUS EN8600GT card in and it worked like a charm!
Now I have the ultimate working machine. 2 x 3.6Ghz XEON CPUs, 8 GB RAM and
an ok card for screen.
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I got to let you know that I have read several places that some cases with doing this
with the sc420 hasnt worked, as the pin number on them are somewhat different, but I do not
know if that is correct. You better check it yourself by counting or seeing if there are pins at the end.