There are some rumors that the Mac Mini may be discontinued and a Mini Mac Pro tower will be introduced.
The current Mac Pro (and the G5 you are considering) are the same size which is rather large. These workhorses or mostly for professionals because they can be customized and upgraded as time goes on. The current Mac Pros have multiple CPUs with multiple cores so they are very powerful.
Some would love to see a mini version of the Mac Pro. Customizable and powerful. The Mac Mini is somewhat of a closed system like the iMac.
Since you didn't mention what you are going to use your Mac for, another consideration is whether or not you need to run Windows applications. The new Intel Macs (which have an Intel x86 based CPU) can run Window XP or Vista natively via Bootcamp -- the same as if they were regular PCs. You can also run Windows via emulation using Parallels or VMWare. Note, with Bootcamp or emulation software, you will need to purchase your own copy of Windows XP or Vista.
The PowerMac G5 on the other hand is based upon the PowerPC CPU. It can only run an older emulation system call Virtual PC which supports up to Windows XP AFAIK. I don't believe that it will support Windows Vista -- but am not sure on this point. Unfortunately, Virtual PC is much slower at emulation because the G5 uses a PowerPC chip which is much different than the Intel x86 CPUs.
So if you need to run Windows applications, then I would suggest the current iMac 24 inch. If you do not need to run Windows applications, then the PowerMac G5 might be a better fit for you.
Again, it all depends on what you plan on doing to do with your computer.