Just adding a data point for the benefit of whoever.
Bone-Stock Dell Dimension 4600 w/ the 5200-series card.
Followed IPC 10.5.6 install exactly as here:
Dell 4600 Install Tips
Started w/ freshly formatted HD, uneventful, except hung on 1st & subsequent boots after install, at Apple Logo/twirling propellor. Would boot fine w/ -f, however.
Reformatted & reinstalled as above, except this time, de-selected all video options for custom install.
Followed the instructions in Post#11 of this thread exactly, using the 10.5.5 core link provided in that message. Thanks astro123.
Seems to be working pretty well, w/ QE, other than what appears to be some type of dialog corruption, in, for example, the title bar of the System Preferences-Network dialog. Not sure how to fix that...
Thanks for all the useful info to everyone who posted here.