What gen iMac do you own? (what year?)
I am currently on a Win7/Mac OS hackintosh plugged into a late 2009 iMac and using it as my display.
These work with Target Display Mode on your iMac:
-Thunderbolt computer > miniDisplayPort display(these were on 2010 iMac's and earlier I believe)
-Thunderbolt computer > Thunderbolt display
-DisplayPort / mDisplayPort > mDisplayPort
-DVI computer > miniDisplayPort adapter (not the crappy ones from apple store) > miniDisplayPort Display
*edit*
Wow 3 pages of "PC vs. Mac" wars and no answer to the OP's question until now..
Get a grip guys. What is "better" solely depends on personal preference.
I like both, so I built a Hackintosh.
As for advice on what to get -
Personal preference but if you can afford the initial headache of setting up - build a hackintosh.
iMac's are basically laptops that sit on a desk. Most of their hardware is the "mobile" alternative (such as the GPU) and generally under perform when you are doing high end gaming & any multimedia editing.
The OS is fantastic as you know, but the hardware is unfortunately a little dated now (as they are beginning to go for "slim and sleek" over practicality.
I am currently running a 4.7Ghz i7, 16GB RAM, GTX 680 4GB on a hackintosh and will probably never go back. The closest thing Apple have to this sort of power are Mac Pro's and they are massively overpriced, and the hardware/chassis is fairly dated.
The one thing you cannot beat with a Mac is their displays - and you have that part already. If you don't need the portability, build a hackintosh, if you do then I guess your only real option is a Macbook (if you wish to run Mac OS).
*final edit*
On a side note, I will mention that the hard part is finding the compatible parts for a Hackintosh when setting one up. The actual install only took me a few hours, and wasn't difficult at all. Some components are easier to set up then others. If you want specific info about this let me know and I will help as best I can.