BenQ xl2420T 120 Hz and MSI Lightning 7970

If your GPU really does not support it, by the specs i've seen yes the lighning version does not support it, you could try with display port cable:

Mini DisplayPort x 4 (Resolution up to 2560x1600)
Single-link DVI-I x 1 (Resolution up to 1920x1200)
Single-link DVI-D x 1 (Resolution up to 1920x1200)

get a displayPort ACTIVE adapter and plug it to your monitor 120hz 1080p should work.

can you tell me exact name or link it on ebay, so I don't buy a wrong one. I was trying to search "displayPort ACTIVE adapter" and I found a lot of options, so I don't know which one to pick ? thanks for help

http://www.ebay.com/itm/ATI-Bizlink-Mini-DisplayPorT-To...

what about this one ? will this work ?

You don't need an adapter your monitor has a displayport connector all you need is a minidisplayport to display port cable.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Yes, when i said adapter i was refering to the minidisplayport to the displayport, but it can be to Dual Link DVI too, since both transfer digital signal.

Go with the one is cheaper/easier to find, make sure it support 120hz.

I got cable mini DP to DP and I have 120 Hz.

I would like to know is there any graphic quality difference/delay lag, compared to dual link DVI ? Is dual link DVI connection better ?

thanks for help guys

No, they are sending basically the same signal, if you are running at 120hz already, you are ok, 120hz at DL-DVI is the same as DP so you are good now.