A couple of years ago, back when MR was still on Vbulletin, I use to regularly browse/post using my 9600/200MP with an unhealthy amount of ram(1.5gb) in OS 9 using Classilla.
I haven't actually tried recently, but I wouldn't be surprised if-with the latest round of updates-it's no longer possible to even load MR in Classilla.
If I had a good Tiger install on that computer, I expect it would be doable. It wouldn't be pleasant, but it would be doable. I have run Tiger on it, but not in a while.
Time permitting next week-after my exams are graded and entered-I might try on my 8600. It does run Tiger(I should get Leopard going) but also has a 700mhz G4(and 1gb of RAM). Realistically, you're probably not going to load MR on any computer these days unless you can run OS X. Practically speaking, whether stock or upgraded, that most likely means an 8500/9500 or newer. If you could coax OS X onto a 3400c you might get away with it(I'll defer to
@LightBulbFun on that) but lack of RAM is going to be your enemy on that. With the big towers, if you don't mind throwing money at them, you can get more RAM than anyone can realistically need on a system that old.