The video card is not as important. OS X Macs did not use the video card to assist with decoding (except for one or two G3s that had DVD decoders) until Snow Leopard and then only when using the 9400M card.720p.. barely. I had a Gigabit Ethernet G4 with a FastMac 1.5 ghz CPU upgrade, 1.25GB RAM, ATi 128MB Radeon 9800 Pro (which is core video supported) and it could not keep up with any 720p video. It would get about 18 fps.That Sawtooth is a good solid box. Most of them can be OC'd to 550MHz or more, and with a fast G4 upgrade and a video card upgrade, will do 720p HD video. Check to see which version of the Uni-North chip your motherboard has. The rev 3 won't allow dual CPUs, but if you have a rev 7 Uni-North, you can upgrade to a dual G4 and NVidia 6200 AGP (flashed PC) video card. Good score!
You need at least Dual 1 GHz (or any single G5) to get good 720p video (VLC and QuickTime H.264 will use the dual processing). At least 133 MHz bus helps a lot too.