Some PC can now work with Mac, but experiences tells me, not every PC hardware is compatible.
From my experience, if you can run Unix or Linux OS on some hardware it may also be able to run Mac OSX. But like I said, not every hardware is compatible even if it can run the above mentioned OS's. Your best bet is trial and error.
There is also now a Dell mini notebook available that you can install Mac OSX on that is fully compatible (a few modification is required).
Please note: Mac OSX will run completely outside of Mac users agreement, and you may not get support from Dell for the hardware, nor Apple for the OS.
However, Apple has already implemented a Mac/PC hybrid that is now capable of utilizing some Intel PC processors that will allow you to run both Mac and PC applications, just not at the same time (Dual Boot Options).
I know you mentioned you don't wish to buy new hardware, but I don't see how you can prevent that if you wish to go the Mac route.
Any how you look at it, you will need some new hardware to be fully compatible with Mac OSX.
As mentioned by "mynameistux's Profile". To prevent from buying new hardware, your best bet is a Linux distros. They all have different usability, so try all of them to see which rocks your boat. And you don't have to install any of the Linux distro to test it, due to a technology call "Live CD".
With Live CD you can test any of the Linux OS without committing a full hardware to it, you can even run it on a PC with Windows already installed on it.
You can download a distros from http://www.linux.com/distrocentral/download-linux or go to ebay to purchase it. Do search on Linux. and get one which is local to you.
You may also get information on the variety of Linux distros here: http://www.shoplinuxonline.com/ you can also purchase directly from there.
If you wish to save a few bucks, and if you have really fast internet connection, download and create a Live CD of any distros for testing:
Hope this helps.
My testing of Mac OSX was not positive, but maybe there's someone out there who have committed to practical PC Mac implementation full-time.
Good luck