Ok, so I actually know what I am talking about, unlike a lot of people who have already posted, so here is my answer.
I use my MacBook Air to game competitively in CS:S. I play tournaments, lans, etc. Right now I use a CRT that is capable of displayed 120Hz at 640x480, and 100Hz at 800x600. In Mac OS X, I only get the option for 85Hz, no matter the resolution. So I believe 85Hz may be the max, but that could be for CRTs since that is the recommended refresh rate by the manufacture. So Mac OS X might be reading the EDID information from the monitor much stricter than Windows. In Windows, I have no issues getting to 120Hz through Mini DP to VGA.
However, I am being sent a review unit of the BenQ XL2420T. When I get that, I will try to update this thread and post how Mac OS X sees it, but I know Windows will have no issues.
So please, wait until I get my review sample to make a decision, and don't base anything off of uneducated assumptions of some of these users who think they know what they are talking about.
I would probably pick up the BenQ XL2420T if I were you, seeing as how it would support DP natively, without an adapter from DVI. That's me though, so just be on the lookout.
You can email me at
ethan@fragboss.com, or check out
http://www.fragboss.com for the review, I will mention it in there as well.