As I write this I am in the process of putting a new drive in my Quicksilver 933.
Quciksilvers ship with a single ATA/66 bus, meaning they can hold 2 IDE drives (1 master, 1 slave). Since the bus is ATA/66, the largest drive you can use is 137GB, meaning that out of the box a Quicksilver can hold 274 GB.
However, there is room for many more drives. I've read where people have crammed 5-6 drives in a Quicksilver (evidently the power supply and the ventilation can handle it).
To put in more than 2 drives, you need:
1) a second ATA controller (goes into a PCI slot). There are several choices - hardware RAID vs standard, ATA 133 vs 100 vs 66, etc. This must be a Mac controller. Check out
Other World Computing for some of the options.
2) A Y-splitter for the power cables. The Quicksilver ships with two free plugs (meaning you can put in three hard drives without resorting to a splitter). The SIIG ATA/133 RAID card I got today includes 3 splitters.
3) A place to put the drives: there are 3 drive bays along the bottom of the case. The rear-most one has the stock drive. There is a drive sled on top of the stock drive which allows a second drive to be installed in the same bay. There are screw holes in place for 2 more drives along the bottom of the case. A fifth drive can go in the bracket underneath the optical drive (assuming there is not a Zip drive already installed). Some people use straps to strap drive number 6 on top of the optical drive.
4) ATA cables to hook everything up. The stock cable has plugs for 2 drives (master and slave). The SIIG card comes with 2 additional cables (it is a 2 channel card), meaning I have sufficient cables for 6 drives.
Good luck! That sounds like a sweet machine!