I have two of these, and they are fine machines, able to take up to 6 drives, plus floppy, and they are excellent performers overall for their age. The PS is massive and very robust — better than the PS that was in the 604e version of the 8600, BTW. The case is identical to the 9600's, with PCI covers and all, and a 9600 logic board can absolutely go right in. However, as the 8600 has on-board video, it already has three PCI slots, and this is enough for almost all users.
On the upside, it has, I think, eight RAM slots, and the 604ev processor is not only fast, but runs much, much cooler than the 604e; one of the nice things about the 8600/300, therefore, is that for many uses, you do not need the noisy side fan, which can actually be disconnected. Mine has run like this daily for years, and for much of that time 24/7. On the downside, the PCI architecture seems to me to be somewhat limited. I have a Voodoo 5500 in one, but performance is not especially gob-smacking.
Mine both had troubles starting up after having been shelved for a couple of years before they came to me. The PRAM seemed to be the problem, and it was a struggle to get them going. Persistence paid off, however, as once I got them going, they have each been pretty much trouble-free.
One still runs 7 days a week as an ASIP server on my home LAN, where it is a nice, flexible, easily serviced, and quiet bridge between the old and the new. I have it set up with a 500GB PATA drive and controller, and a couple of 9GB scsis and controller. I have the drives set to spin down when not required; it is almost silent with only the PS fan turning. The other was used for about five years by one of my kids, and though these days it is much neglected, for a long while it got a lot of classic gaming use.
Back a few years ago, I tried OSX (X.1, I think it was) briefly on a stock 8600/300 604ev via Xpostfacto, but the trial was let down by the relatively weak on-board video I was using. In those days, I did not have a decent video card to try in the thing under OSX, but it occurs to me that one day I ought to try it again with a Rage 128 PCI card and just see what happens.... Multi-threading in a 604ev should be better than in a G3.