I believe a good place to start is with a decision: Do you want to keep the stock G5 look, or do you want to replace the back with an ATX kit, like those available from The Lazer HIve? This is all a matter of personal preference. I like the look of the case as-is and didn't want to change it. Both require some work to get what you want, but each has its own rewards and challenges.
Once you have that figured out, build from the inside out - by that I mean get the Motherboard, CPU, PSU, GPU, HDD/SSD, etc. that you want. Again, a choice needs to be made here: Go Gigabyte and nVidia for run-ability out of the box, or go with the system that you want and try to make it work with OSX. Some people will tell you to go the easy way... and that's fine. I can, however, tell you that its only easy because someone else did it first, and found out how to do it, and then shared it with the community. I went with an ASRock board and an ATI video card. Some things didn't work right away and I had to tinker - but that was part of the fun for me.
There is a guy who built a hackintosh in 90 minutes and posted the video on YouTube. And there are people like me who are still tinkering with their system after more than a year.
This site is a great reference for both the hardware and the software aspect of getting your hackintosh running. Check out the mods here in this forum, and when you're ready, move to the top of the page for software help.
As always, don't be afraid to ask...
... and don't be afraid to start modding your system!