It really depends on how long the motherboard has been out and how recent the bios have been udpated.
Example... I.e.: New motherboard to the market will come with the latest BIOS but a motherboard that has been on the market for a few months and a very recent BIOS has been updated, it won't becoming with the motherboard.
Depending on your MOBO and CPU, it will be likely to boot up even if not supported. It'll boot up as an Un-supported CPU until you flash your BIOS.
Can you list your motherboard and CPU to give us a better idea?
You might be lucky with your setup for the II X4 925, as there have been two BIOS updated, since it's release. The II X4 640 has had one BIOS updated since it's release, so you might be lucky, as well.
Other words, you could be good to go without issues.
With most motherboards, If you plug in a newer CPU that has not been supported from the first BIOS, You can boot up with the CPU you will get the message 'Unknown CPU is detected, updating BIOS is required to unleash its full power! Press F1 to continue'
That happend when I installed my x3350 into my P5B Deluxe-WiFi/AP. Windows booted fine and I downloaded the BIOS and updated using EZ Flash2 in the BIOS.