Greetings!
First time poster here -- this forum seemed like the best community to discuss my questions

So I'm kind of in the same situation except I have a 2012 11" MBA. I've been considering an upgrade to a Haswell-based computer due to the obscene battery life, and I'm on the fence between a 2013 13" MBA and the new 13" rMBP.
If I went with the MBA, I'd get the one with the 256 GB SSD and upgrade to 8 GB of RAM, then add AppleCare on top as I foresee keeping this one for a while.
The "middle" 13" rMBP already has 8 GB of RAM and the 256 GB SSD, and if I add AppleCare on top, it comes out at about the same price as the decked out 13" MBA I just described.
So it's not really a price issue, more of a specs issue.
My main thing is I'm a gamer. I know, I know, I probably shouldn't get a Mac, but I want something light and portable and I like OS X and I can install Windows easily. I'm 100% willing to compromise on gaming performance, meaning I don't need to run my games on super ultra, nor do I expect to run demanding games in the first place. I have a high-end desktop for my regular gaming.
The rMBP appeals to me because it has Iris, which is quite a bit better than the MBA's Intel HD 5000, but it's not able to play games at 2560x1600 (even high-end desktop cards would struggle), so...
After stating all of the above, my main question is: how do games run on the rMBP at lower resolutions, whether in Windows or OS X? I'm perfectly OK running games at 1440x900 for example, or even 1280x800. What I'm wondering is if the device can "handle" that in terms of letting me pick those lower resolutions in games, and how they look and play. On the MBA, I'd be running them as the screen's native resolution so it's a non-issue, but I'm wondering how the rMBP behaves in those situations.