Your very FIRST step: using CPU-Z, find out what your RAM timings are for 333MHz and 400MHz. Print these out - or write them down, note them on your cellphone, anything as long as you have them handy when you go into the BIOS again. Do the same for CPU core voltage.
Reset the BIOS. Completely to factory defaults. Incidentally, starting the motherboard without a CPU installed resets EVERY piece of firmware on the board to factory default. This trick has been used several times to 'resurrect' a seemingly completely 'dead' board, depending on what killed it.
Right. Step 1: In the BIOS, set your RAM to a 1:1 ratio. This way 333MHz on FSB = 333MHz on RAM. Gives you some breathing space. Also, set every RAM timing you have values for - use the 400MHz timings. Set the CPU voltage to what CPU-Z reported it as - basically, in the M.I.T. screen you want nothing on 'auto' and as little as possible on 'default'. Just don't actually overclock anything yet. You also want SpeedStep/EIST 'off' and C1E 'off'. In fact, any power management options should be 'off'.
Step 2: Boot into Windows. Check the settings in CPU-Z to make sure it applied the changes and is holding them. If you want to, run a stability test and/or benchmarks to establish a baseline.
Step 3: Go back into the BIOS and up your FSB by 5~10MHz. Repeat step 2.
Keep repeating steps 2 and 3 until the system goes unstable. Then you move on to
Step 4: Go into the BIOS and increase the CPU voltage ONE increment, TWO at the ABSOLUTE MOST. Do NOT perform this step UNLESS you have a large heatsink or water cooling. After this is done, go back to step 2. Lather, rinse, repeat.
If even this does not work, you have severe problem(s) somewhere. Check your PSU and make sure that both DIMMs are identical. How much power can your PSU deliver? If problems persist, post your entire system specs - this may help in pinpointing an 'obvious' problem.
Just FYI, I achieved 5.2GHz on a 3GHz P4 chip (200x15 up to 347x15), on the P35-DS3L (the earlier version of what you have), and am currently running the same board with a Pentium D 2.8GHz running at 4.2GHz (nicknamed the 'Baby Furnace' because I had to upgrade the rad TWICE), this is fed from a 750W PSU. Even watercooled, the 5.2GHz P4 was hitting 72ºC, and the current incumbent can hit 67ºC.
This was also the board where I developed a major hatred toward the E4x00 series of C2D processors and a brief dissatisfying affair with an E5200.