I know this is a pretty old post (2011), but if this helps anyone...
I have a Quicksilver G4 I have been converting into a hackintosh. I am currently running a Gigabyte GA H61M USB B3 (3 Gbs SATA) which fits perfectly for the back port positions, as the audio ports are quite far away from the corner screw hole along the back edge, and very little cutting is necessary to make it fit, although you may notice that the Ps/2 ports may not fit, but I chose to cover these as they are not needed. If you want a more current board (with 6Gbs SATA) in an easy to modify case then get a GA-Z77N-WIFI and use a G4 Graphite case, which has a wider area for ports. Basically any Mini ATX board will fit nicely in the graphite case, but it will not look as nice as a quicksilver or a G5 case. On both G4 cases you must remove several of the existing stand offs and drill new holes for 10MM stand-offs, which is not very hard to do. (The stand-offs pop out if you twist them out with a circular motion with a needle nose pliers or vice grips.) If you get m-m threaded stand-offs, you can adjust the board while in the case, and tighten them from the bottom to achieve the right alignment. I use a dead Mini ATX Board, to align and fit the holes, to prevent damage to the Gigabyte, and any board will fit perfectly once you have all the stand-offs in place. Use locking screws if possible.
With either G4 case you can use an Antec 500W Basiq power supply, it fits the same, but the power switch has to be popped out and/or moved, or you can cut the plastic on the back to fit the switch. Some builders have suggested taking the guts of an Antec or similar power supply, and modding the original P/S. If this is possible it will look the most genuine to the original. I plan to move the power switch to an accessible location on the bottom side of the P/S so it is inside the case and cannot be tampered with when the case is locked.
Another tip, is do not use the shark-tooth fin memory, as it is too tall, and will hit the Cd-Rom drive. I use Kingston Hyper-X Blu, and they are fine and match the overall color of the board too.
Aside from only having 3Gbs SATA (Sata II) the GA H61M USB B3 has performed flawlessly for me and I like having the older PCI slots available. (It does have 2x USB3.0 ports) If you go with the GA-Z77, you will lose some pci slots, and have only one slot x16 for the video card and replacing the Wi-Fi card on the board with a Mac compatible card will probably be necessary. It would be nice to use a full size board in these cases, but there is no room in the G4s, so if you want all the slots, look into getting a G5 case to modify. The plus of both of these mini boards is that they use the Realtek audio chips, which is easier to deal with with kexts.
I hope I can document this build at some point, but it is hard enough finding time to work on it, much less take pictures as I go. I opted not to go with a G5 case, for one other reason, the Older case, is less likely to catch someone's eye during a break-in, they may assume it is an older Mac, not worth taking, (plus it's very heavy.) I also found upon taking apart the quicksilver case that there are fixtures on the back of the plastic panels, behind the apple logo that look like they were designed to hold an LED, so I plan to install White led's to make the apple logos glow when it is on. Kinda cool, I wonder why they never bothered adding the lighting.
I really recommend too, taking apart a heavily used G4 case and thoroughly giving the case and all plastic parts a bath with with pine-sol and detergent to dissolve smoke tar, and other dirt that could capture more incoming dust, before enclosing your motherboard. I was surprised how filthy mine was, and this makes it easier to dislodge dust later-on with compressed air on periodic cleanings. And the hinge work smoother too.
P.S. I am working on a G4 graphite case and painting the inside of the transparent panels with a dark navy blue. I never much liked the transparent panels on the graphite, and initial tests indicate that it will look very unique. I will try to post some pictures once it's finished. I suppose a silver paint might look cool too, but the plastic is not clear colored like the quicksilver.
Gigabyte GA H61M USB B3
Intel i7-2700K 3.59 Ghz
16 GB Kingston HyperX Blu RAM Cas Latency 9
2 - Seagate Momentus XT 500 GB Hybrid HDDs
Gigabyte Radeon HD 6850 GV-R685OC-1GD
LITE-ON iHBS212-08 Blu Ray Burner (formerly Sony BWU500S - R.I.P 2012-2013.)
Sony Optiarc AD7280-S DVD Burner
Antec Basiq BP500U 500W Power Supply
Note: Use locking SATA cables on all drives if you plan to open the case often, as they can fall out easily with movement.
