New HD in 800-17TFT-g4-Imac >how big???

Hi,
I´d like to swop the old 80GB harddisk of my800mhz g4-flatpanel imac ( OS 10.3.8) but I don´t know how big the new one may be. I´m thinking of a Hitachi 250GB one.
Apple hotline would not give a secure answer --or could not..I don´t know.

Maybe one of you has already done such a change and can give a hint.

thanx!!!

hi,
yes, I know about the difference between 2,5 and 3,5...

it´s the gb-size what bothers me. i´ve heard of the limit of 128gb.
my imac has a busspeed of 100 MHz. the hd should be an ata100.

last year i had found an internet page on that problem with bigger hd and how to put them inside the mac. but i lost that adress.

nevertheless thanks!