Now 333mhz is mostly 2.5v which is fine, but 400mhz stuff starts at around 2.6v, yeah it should work alright. Because the ram will be running lower than its rated speed, so it should require less voltage.
I should have said that my motherboard only supports up to PC2700 RAM. The PC3200 DIMM is therefore running at PC2700 speeds. I went for the PC3200 DIMM because it was the same price as the PC2700, and I thought it might be more useful if I ever buy (or even build!) another PC in the future.