Capt Fiero is generally correct, IMHO.
I've flashed my BIOS' (four boads, total) many times using all three methods, including the internet method (AsusUpdate). As long as you've got a reliable internet connection (preferably BroadBand - less chance of noise/drops), and a reliable PC, and power to your PC, you should do fine. I'd suggest the US Asus.com server if you live Statside, and the German (.de) server if you live on the continent. I've seen some intermitantcies when attempting to connect w/the Taiwan (.tw) server sporadically (at least for us in the Western Hemisphere). I've used the internet BIOS update Utility at least 6 - 8 times w/o a problem, though I generally use, and feel safer using, the 'tradtional' floppy method.
Just launch your AsusUpdate Utility (it was included on your Asus P4P800/P4c800-E CD - The same CD as your INF Drivers, AusuProbe monitoring utility, etc.). The rest will be GUI oriented, & pretty intuitive. If you've got the AsusUpdate version from the original CD it will probably 'notify you' it needs to update itself (automatically off the internet), before you proceed to d/l & install a later BIOS.
Again, this is all very intuitive, IRL, and much easier than it sounds when explained in words. Load the AsusUpdate from the CD, if you haven't yet done so, & launch it. You'll figure out the rest - It's that simple.
HTH
GL
Strat