New
#761
If the "Memory" tab in CPU-z shows 533MHz, then that's what you RAM is running at... The "SPD" tab shows the compatible speeds according to your RAM, while the "Memory" tab shows the current settings. Besides that, according to Gigabyte's website, your P35-DS3L supports DDR2-1066 (link).
That could be for a number of reasons; have you tried increasing the voltage supplied to your CPU or RAM? Overclocking makes a component work harder by increasing it's clock cycle rate, therefore drawing more power than normal.
Although, seeing a review I just found now (I was waiting for the page to load while I was typing this), it appears your motherboard doesn't like overclocking that much anyway... Have a look here. Specifically regarding the point that they increased the CPU voltage to 1.5v and well.. Melted the plug. Lol... That must suck.