Are you looking for storage space or preformance? If you're looking for storage, redonkified is right. If your looking for preformance you'll want to install a Western Digital Raptor. Here's a link http://www.storagereview.com/WD1500ADFD.sr
With regards to performance, do remember that hard drives on ALL G5 powermacs only operate at sata1 (sata 150) speed. Most drives produced today are all sata 2, but the interface connections are the same. On some drives you can change the jumpers to downspeed them to sata 150, but in my experience it's not needed. The G5 will just work at sata 150.
My choice of drives for my G5s has usually been Western Digital (great performance and good value) or Seagate (a little more expensive but quiet). Both are reliable. I did have a bad experience with a Maxtor, but maybe I was just unlucky.
As has been mentioned, "any" SATA internal will do the trick. I just picked up a 500GB Seagate for just a bit over $100, which I'll be using to replace a 160gig.
I don't have a plan yet for the 160 I'll be removing; if you're close to NJ and think you might want to offer me $25 and pick it up, PM me and we'll sort something out.
In general, the only problem that I had installing my first SATA drive in a Mac was to figure out how to sort out the details of sliding it in the bay. I initially thought that Apple didn't provide the cable hookups, but they are there: its merely neatly tucked away. Patience and good eyesight...the solution's all there, you just have to look for it.