homework
my only advice - do your homework. there's lots of reports out there letting you know in black and white what cards can be sucessfully flashed, as well as those that can't. don't get yourself in a pickle by assuming a card can be flashed, cause there's a good number that cannot.
if you find a card that can be flashed (like just about any matrox card ever made), and a flash utility that seems legit (most are, so i wouldn't really worry about that), then you really don't have anything to worry about. just follow the instructions.
i just recently flashed a matrox card... nothing major, just a little pci job for my beige, and i couldnt' be happier - matrox has their own flasher utilities and bios on their site, so it was no worries...
good luck!
matt