OK first off, there is nothing wrong with your video card memory. If anything its your system memory or motherboard, but for the time being lets assume they are OK and try something else.
You said you have the power connected to your 6600GT, now what I want to know is did you use a dedicated molex line from your
PSU to power the card, or did you use a shared one. Most Molex cables have 2 or more piggyback joins, so that one cable comming out of the
PSU can be used to power multiple devices. For stability, and always better overclocking results, use a DEDICATED molex from your
PSU, that is not plugged into anything else at all.
If that is already the case, then you might want to try another driver. The best driver package for you to be using is the 76.50 Valance Studios / Xtreme G modified set, which you can download from AOAFiles.com
Next, try removing one of your memory sticks. You are on a S754 board from memory, and they are always more stable running 1 stick than two, particularly at high speed. The fact that the memory can run at its stock speed, doesnt mean that the S754
CPU's onboard memory controller can handle TWO of those sticks at the same time, and they are already fast sticks, so I assume that could be your memory problem.
Remove 1 stick, and it should not have any memory errors, and if you want to run two sticks, use a 2T command rate, and reduced speed (166Mhz probably).