Based on googling '(vmware OR virtualbox OR xen) (osx OR leopard OR
tiger OR "os x")', there are a number of legally ambiguous (read:
illegal) ways of getting Mac OS on plain 'ol Intel hardware.  The gist
is that you need:

1) A copy of Mac OS, patched to remove any pesky checks that only work
on Apple hardware.  This is one of the more legally questionable
parts, as you're ostensibly downloading said copy of Mac OS from a
torrent site (thus sharing your ill-gotten gains with others), and
because monkeying around with the OS code itself (i.e., "patching") is
a giant legal no-no, at least according to Apple's EULA (well, I
assume so... has anyone, in recorded history, ever read a license
agreement?).  It doesn't look like a stock OS X install disk will
work.

-- and --

2) Pretty recent Intel hardware.  Your CPU needs to support, IIRC,
SSE2, SSE3, and probably a bunch of other fun initialisms.   So you
probably need at least a Core2Duo, or something along those lines.

On the other hand, if you have a spare Panther license, and some
patience (and, again, the will to brave legally murky waters), pearpc
would be an option.  I believe pearpc will work with a stock install
disk.  It's not the latest-and-greatest Mac OS, but if your goal is
tinkering with an Apple UI, it'd be a good start.  Of course, you
could always just buy a Mac...

As for virtualization in general, I've had good luck with VirtualBox
when I've been stuck with a Windoze-only app for whatever reason.
Performance-wise, I don't think you'll see much difference between any
of the major virtualization apps.  The main differences would probably
lie in the ease of allowing the guest OS to see your external devices
(e.g., the iPod+iTunes -- hey, Apple again! -- combo can be tricky).
Google would be your best bet in those cases...

Good luck,

sm