Classic won't run on the Intel version of 10.4, and to my knowledge, there's no way to enable it (in fact, it may even be impossible since Classic requires certain PPC instructions which don't exist on Intel, even with Rosetta).

SheepShaver is adequate for some things, but it's relatively tricky to st up, and very fickle (it likes to crash a lot if you don't configure it (and the emulated Mac OS install) very carefully). Also, it's only limited to 9.0.4 because it doesn't emulate an MMU.

There might be other solutions out there, but from what I've seen, SS is the most viable (for now) for classic PPC software (I think some (most?) Carbonized software WILL run on Intel 10.4, with some caveats, because Rosetta is capable of supporting them).

And of course, if you want to give it a try (and don't mind being limited to 68k), you could get the Mac II version of Mini vMac and run System 7 on it. It, too, is rather buggy and incomplete, though (but it has the advantage of having major new features, such as more complete emulation, being added on a semi-regular basis).

I hope this helps?

c

EDIT: Of course, if you only want to run OS X by itself, then it should actually work quite well. I recommend machines based on some variant of the Intel GMA 945 chipset, with which 10.4 is most compatible (in my experience, anyway).

I also recommend using at the oldest a late model Pentium D CPU (ideally, you should use some sort of Core2Duo CPU). Although hacks have been developed to support older processors, such as the Pentium 4, I prefer to err on the side of caution and only install it on computers which have all the required instructions built in, rather than relying on potentially buggy hacks and workarounds.

EDIT #2: I agree with Gorgonops. He makes a lot of good points (which I have repeated here).