The iMac uses a standard desktop PC hard drive. It is 3.5" wide and in the iMac G4 uses the IDE connection. The iMac 700MHz 15" has an ATA66 bus so can use ATA66, ATA100 or ATA133 drives but not ATA33. You should buy a 7,200rpm drive with a cache of 8MB or higher as this also increases the speed.
It's very fiddly to work inside an iMac G4 so you must be cautious. Always take anti-static precautions. Here are three options for guiding you through the procedure:
I've also taken a fair few photos which you can find here.
I'd advise reading the reports on the xlr8yourmac's drive database so get other people's experiences first.
You must replace the Thermal Paste when you close the iMac up or it will overheat.
It's probably worth replacing the PRAM Battery whilst you're in there as these have a finite life (once they fail to re-charge you may start to run in to snags - it's only a $5/£6 part).
The hard disk should be set to Master (link).
You don't need any additional software; SpeedTools allows older iMacs (including some iMac G4 700MHz) that are unable to address hard drives larger that 128GB.
You might also give consideration to installing a 512MB DIMM or an 18x DVD±RW Dual Layer burner whilst you do this update; both of which will have to be removed during the process.
You must be very careful around the video connector in the iMac; it really is very fragile. In fact there is no need to touch it at all in my experience.