Yes, flashing the BIOS incorrectly can result in a total system frack-up. However, if you take the necessary precaution there is little risk, and no more so than having it done professionally.
The main risk when flashing a BIOS is that any discontinuation of power to the motherboard will result in bricking. If you live in a building with faulty electrical, or happen to be toying with your computer in the middle of a lightning storm, please don't flash your BIOS. Otherwise, your chances are pretty good.
The flashing itself is relatively simple: you need to know the EXACT model of your motherboard, and the EXACT BIOS you're flashing to. Most major motherboard manufacturers have a few different file types to flash with (you'll find instructions on the website), but the principle is the same: load the file onto a floppy (it's not a boot disk) and run the flash from BIOS. Wait 5 minutes, let the computer reset, and you're golden.