Bump Vcore up to 1.45, FSB to 273MHz, 1:1 memory divider with 5-5-5-15-2T timing, 11x multi, and reboot PC. Run Orthos for about 5 minutes. If okay, then raise FSB by 3MHz and retest with Orthos Large mode. You'll probably run out of gas around 310 MHz FSB. Once you're happy with Orthos, run Memtest86 (test #5 and #8) for 2 hours each.
You can also overclock RAM if you have quality DDR2 667 or higher speed. Bump Vdimm to 2.0V and raise memory divider to 1:1.20 or 1.25. Run Memtest to check for stability.