+1 on the recommendations for an air cooled G5. Story time.
I had a 2.7ghz G5 powermac that recently kicked the bucket. It had leakage from the ICS on both cpu's, as well as damage to the (logic board?) motherboard. It was unfixable on a computer I bought for $50. So, I picked up a 2ghz air cooled G5 and the morphos team gave me a new keyfile at no charge (thanks!) so I'm currently using that.
Initial thoughts: Most things _feel_ like they are the same, 2.7ghz vs 2ghz. There is a pretty stark difference in OSX, though. The only time I really feel the slower clock speed is actually with OWB browsing the web. Specifically, opening new tabs takes ages. Other than that I didn't really notice the difference in clock speed, even with gaming.
Now, I personally wanted MorphOS to be more or less a solution for me to do all my retro Amiga stuff on newer and faster hardware, in particular run PPC software. To a degree, I can now do this. However, Warp3D is only available on some gpu chipsets. I have a Radeon9800XT in my machine, which is fantastic for games designed for MorphOS, and ports of games like doom and quake. It's seriously fast and awesome. However, I can't run WipeOut2097, Shogo MAD, or those very few games I intended to run in the first place
Dual GPUs don't work for me, though I guess on some G4 powermacs they do. The dev team has no intention of bringing warp3d to any other cards - they look at it like it's dead and not useful.
I do have a radeon9250 (flashed for Mac) that I've successfully booted and run the aforementioned warp3d games on. However, it crushes some of my other games that I really enjoy, so it's not useful. Ideally, I would like to be able to run dual GPUs, my 9800xt in agp and the 9250 in pci.
One other talking point is that the G5 doesn't really have much "optimized" to run on it. UAE runs like absolute garbage for me, so whdload games aren't currently an option. Which...really sucks. Oh well.
My recommendations depends on what you'll use it for. Warp3d games and stuff: Find a mac mini g4 with silent upgrade (double GPU RAM) and you'll be happy! Want a more robust system with potential to later support more ram, different GPUs, etc? Get a g5 air cooled powermac.
