The original Quicksilver was the last PowerMac which didn't support large drives. Intech Speedtools can allow use of larger drives on unsupported Macs. I have installed a 750GB HD in a Cube.

look at youtube..

I have seen people take g4 towers and g3 towers and crank those mothers up.

I saw one where the user placed a huge watercooled heatsink in a g3 tower talk about insane.

This is a blog dedicated to quieting the MDD. I have several posts there as Jeff B.
http://coolstuff.wordpress.com/2007/02/19/powermac-g4-mdd-cooling-and-noise-reduction/

Its the best pool of info I have seen. Used all SilenX in mine and plugged the 120mm replacement into the mobo rather than standard 4-pin psu power. That way the fan only gets loud under heavy load but even then its still much more quiet than it is on a 4-pin. On the 4-pin it will always run at full speed. I did plug the 60mm psu fans into the 4-pin because the new fans only blew 18 cfm vs. 35+ of the stock fans. I also added a 3rd 60mm to the back exhaust of the psu to help suck out more heat. Never had a heat issue. PSU always feels more cool than warm and is very quiet. 16dba.. anything under 20 is inaudible if your tower is on the floor.

What kind of hard drive do you have in your machine? SCSI usually create a lot of noise. I have the same G4 Quicksilver and them most noise is created by its SCSI HD. Barely hear the fans.

I agree about checking out YouTube as there are several videos regarding modding G4 Towers.

Youtube has been well known for resolving issues relating to such as the one your experiencing try searching it in Google, I am sure someone has had this problem before and has made a video to fix it.