I assume the number '1536' after RAM in your sig means you're running 3x512MB. If so, though you may not want to, removing one of those sticks will definitely help stability. Just because the board is capable of managing 4GB of RAM and any number of combinations of lesser amounts doesn't mean it will hold a stable overclock with an odd memory configuration (3 sticks). You won't find too many sigs advertising memory configs other than 2x256 or 2x512 combined with high system clocks. That's not to say it can't be done, but your chances of a higher stable OC are better with 2 sticks. You may have also reached your CPU's stable limit, unfortunately. My recent 3.2C isn't a great OCing chip; while it will post at 4GHz, it's not stable at any speed over 3.8GHz regardless of vcore. Recent Northwoods have shown mixed results when OC'd; I haven't had any real winners myself. BTW, random reboots can often be cured with increased vcore, in case you haven't already tried that. Good luck.
