My view on the computer upgrade has changed in recent years. I used to advocate and personally did a lot of upgrades on my home computers, CPU, videocard, memory, motherboard, disk, videocard, you name it. I don't do that any more. I don't think it's cost effective.
In the earlier years, the computers were very expensive. (I'm talking about 10 years ago) You don't just go out and buy a new machine when it breaks down. You fix it and upgrade it, and pamper it so it can last a very long time.
The computer has become more of a commodity. The computer price hasn't really gone up. Considering inflation, the relative price actually came down! Now everyone has one or more computers. If you go to eBay, you can buy previous generation computers very cheap.
To make the long story short, I'd rather sell my computer on eBay and get a faster one than trying to upgrade my old computer. I'm sure you are very familiar with this process as you mentioned you bought yours on eBay.
If you take old computer and try to upgrade various components to make it faster, you'll realize that it's actually cost effective to sell and buy a faster computer. That's partly because the sum of components cost more than the computer itself, and there are a lot of components.
For example, you have a dual 500MHz. It has 100MHz bus with PC100 memory. (or PC133, but my point being it's older technology) It has maybe Rage 128 or NVidia MX2. It probably has DMA33 or DMA66 disk controller with 5400RPM drive.
The next generation, mirrored door PowerMac G4, has DDR memory, ATI Radeon, and probably a lot faster drive. The thing is, each components are just little bit faster than the previous generation, but they add up. The combined performance will be VERY noticeable.
If you try to upgrade your Dual 500 with CPU, videocard, and disk, you'll end up spending maybe $500. But if you look at the used computer price on eBay, the difference in system price between Dual 500 and mirrored door is probably less than $500. Plus, the newer system will still be faster than heavily upgraded sawtooth.
Since you are already set to upgrade the current computer, just keep this in mind for the next time when you buy a computer. It's very easy to get carried away in upgrade frenzy, and end up spending more money and more inferior system.