I agree with marvin, as you will have to go 2 steps:
from pix->ASA 8.2
from ASA 8.2-> 8.3/8.4
there is going to be a fair amount lost in translation if the config is complicated. and you will have to review it extensively anyway. if its easy config, it will be easier to rebuild it from scratch.
as far as the cutover (if its allowable and somewhat simplistic), try to have both devices in line and turn one off and the other on. then you can fail back easier (don't forget to clear arp on the related uplinks).
if not allowable, might want to create a link between PIX and ASA and do an "inch-it-in" upgrade, where you can test things as you go.