The question is whether that BIOS upgrade simply allows the e8400 to work, or if it actually allows that motherboard to support an FSB of 1333. If the BIOS upgrade doesn't actually improve the FSB ability of the board, then your E8400 would be forced to run at 1066 FSB instead of its native 1333 FSB. So ultimately, it would slow the bandwidth ability of your CPU.
Ulimately, you want a motherboard that will support the CPU's native 1333 FSB for the best performance.
You'd essentially have to Overclock the Mobo chipset to get the 1333 FSB the E8400 is made for (with your current board). Which means you wouldn't have any room for actually overclocking your processor (and possibly cause extra heat from your motherboard chipset).