Brief history lesson:
On June 6, 2005 Apple opted to switch from using the PowerPC architecture processors switching to Intel.
Prior to that time all Mac computers ran on Motorola or IBM PowerPC processors.
Even though the PowerMac computers of that time were capable machines, you will be FAR better off buying a used Intel Mac Mini which will be more capable yet.
One thing you need to know about the tower G5 machines is that the liquid cooled models tended to leak (I am an owner of a G5 tower that leaked).
One other thing you need to know, G5s run HOT (cpu temps in the 180 degree range under load, backside of the memory controller as high as 213 deg fahrenheit). It serves as a room heater in winter. In the summer it can be unbearable without air conditioning.
One other thing you need to know, G5s use a LOT of electricity (my electric bill has been excessive for the last five years I've owned the thing - over 350 watts in regular operation).
The repairs for the leak were VERY expensive and I had Applecare so I didn't have to pay $3000 for the repair. Next up, the power supplies tend to be brittle. I'd hate to see you buy an old, obsolete computer only to be dead in the water in six months.
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A Mac Mini will have more horsepower than the G5, and will generally be a more capable useful system with faster memory, faster processors (and fabbed at a smaller nanometer) all around better.
The other downside of PowerPC processor Macs: You are stuck at 10.5 Leopard. There is no Snow Leopard upgrade for PPC machines. There are software packages you WILL NOT be able to run.
Want the latest version of Logic Studio? The premiere Mac based digital audio workstation application? Forget it, it only works on Intel processor Macs with Snow Leopard. My G5 has 3 TB of disc and 8 gigs of ram and a super hot video card but it can't run it. It runs the last version but that's not current.
Some more examples - want to stream a Netflix movie? Only works on Intel Macs under Snow Leopard. Doesn't matter how fast your processors are or how much ram you've got, you're locked out.
Want to run Steam Mac games? Only available on Intel Macs. Same problem.
Get the picture? Do yourself a favor and seek out a 2-3 year old Intel based Mac if price is a problem and go from there. Or better yet: seek out a NEW Mac at the online store at apple.com and look at the reduced prices of REFURBED machines that make terrific values.