Mac Pro (early 2009) with ati radeon hd 4870 - Monitor Resolution?

Don't know if anyone has the answer . . .

We use an early 2009 Mac Pro, with the Ati Radeon HD 4870 video card. (This isn't about upgrading . . we need to run OS 10.9 for our recording software.)

Our monitor died and we were about to buy a new one when and interesting question came up about monitors. We were about to by a 27" Acer monitor but a few complaints were that this monitor, which is rated up to 120Hz, was only running at 60Hz on an number of computers. Acer's response was that the video card has to support the 120Hz but I can't find any information about whether our video care, the Radeon, CAN support that high. We sometimes need to do some video work so the 120Hz refresh, or even something like 100Hz, can make a difference, over the 60Hz.

I contacted Acer but they said to contact Apple for information.

Anyone have any idea of how I would get this information? Finding the specs of the card is easy but nowhere does it say exactly what it will drive and it would be great to have this knowledge before buying.

Thanks in advance for any advice.