You want the best of both world? Here's some advice:
1. Build your own Windows PC, say for $1200 you'll get a nice powerful tower for years to come. For hardcore gaming, video editing and so on. Don't forget to include SSD though. Again it's much cheaper than configuring SSD on the iMac.
2. Get a 27" display to match 27" iMac, say something like Dell Ultrasharp 2713HM, it's a 2560x1440 and you can get it for around $700 or less. It comes with multiple video inputs for flexibility, and USB 3 hub too, so it's a nice monitor compared to ATD. Not to mention it has 3 years warranty from Dell. Perfect!
3. Lastly, if you have some more $$ to spare, get a Macbook Air, a 11" high end model would suffice for around $1100. You can connect it through DisplayPort to your monitor. Voila, you get a Mac, and mobile computer to be on the go.
Those configurations will be perfect and versatile for any condition you'll likely encounter. Be it heavy editing, hardcore gaming, or lightweight stuff on a Mac. You hardly miss anything