the setup mentioned above is just the mounting you'll need for more HD's.
I always thought the G5 has the SATA already. Meybe the early G5s didn't ??
Anyways, by using SATA card, you can use now SATA HD instead of the old ATA 100 Bus standard. This indeed eliminates HD restrictions. By using SATA you also have access to a lot faster drives, especially the 10K Raptor 150G dirve. Having this a start dirve is pretty nice setup.
Regarding model or brand, I would go with Sonnet or Seritk.
For my dual 2.5 PowerMac, I got a Venus external SATA tray that holds 4 hard disks (on sale from Frys for $120). I connected it to a card I bought from CWOL (PCI-x) and find it works perfectly. I have 4 disks in the tray and have turned them into 2 disks using striped RAID on OS X. The tray sits on top of the PowerMac and does not take up much space -- it still fits under my desk. It is quite silent and seems to me a much better solution than trying to cram another disk into a G5 PM. It is VERY fast.