I want to run Leopard on my pc. How possible/smoothly would it run on these specs:

Intel Celeron 2.66GHz
1.25GB DDR RAM
256MB VisionTek Radeon x1550 PCI

Anything else you need to know?

On a PC? Don't you mean your mac? It's illegal and difficult to run Mac OS X on a PC.

If you're going to run it on a mac, then yes, it would run very smoothly. 1.25GB RAM is an unusual number, but I don't suppose it'll do harm. 2GB is a nice add-on for multitasking but it's not necessary.

~Knight Haleth

Actually I just double checked the license agreement with apple, your not allowed to run OSX software on anything other the Mac hardware. That includes VMware and such. Its a shame really that apple have also completely closed off support of OSX server too, the extension only allows it be installed under a virtual environment on a Mac. I'm curious as to why this is?

No, it's not very hard. You can run the open source version of OS X, called Darwin (it's the exact same thing) on PCs. It will need to be patched so EFI doesn't get in the way. I can't give you download and installation details, seeing that would be against the rules, but by Googling "osx86" you'll get pretty far yourself probably.

On your hardware; it'll probably run smoothly. Unless you run it in a virtual machine, like me, which is slow but easiest and safest. A patched version probably won't support dual core CPUs, so a 2.66 GHz single core is ideal.

Although I don't recommend Leopard when it comes to running OS X on PCs. Tiger is a lot more compatible, and a lot more things have been patched up properly to work on non-Mac computers.

Note: EFI is Apple's main way of making sure OS X is only ran on Macs, instead of PCs.
Note 2: Not all hardware is supported. My motherboard, for example, isn't and that renders it impossible for me to install OS X on a harddrive. In a virtual machine, however, the software works fine. But it is very slow.

Well, since it's illegal. This thread can be closed if it needs to (like me not giving mods permission is going to stop them, lol).
If it doesn't, I don't exactly plan on installing it anymore, but I just want some more information.

Based on what I've found, I can install 10.4.6 on my Optiplex 170l, but I can't install more recent. Would I be able to at least update?

CPU-Z dump in spoiler.