A dual link port can run 2 monitors if they have a resolution of 1920x1200 or lower. So if the second monitor is going to be another 20", the existing card CAN RUN BOTH. 😀 You'd need to get a DVI Y Splitter. 😉 The port will run BOTH monitors, if you mean to do it off the same card.
If not, then the 3870 would work (intending on a separate graphics card for each monitor). OS X seems to be able to handle cards by different manufacturers, but Vista won't. If you want to run windows, you'll have to wait for Windows 7, as it doesn't have a problem with mixed graphics card makers. In the mean time, you'd have to pull one I believe. (Search the forum, as there's info on this).
I'm not sure if you want a second card (separate card for each monitor), or the size, so I can't really offer any more information ATM.
A dual link port can run 2 monitors if they have a resolution of 1920x1200 or lower. So if the second monitor is going to be another 20", the existing card CAN RUN BOTH. 😀 You'd need to get a DVI Y Splitter. 😉 The port will run BOTH monitors, if you mean to do it off the same card.
Actually, I was thinking in as general a terms as possible, and figured the monitor would likely be DVI, and going with the Y splitter might avoid some issues users are mentioning about the Mini DisplayPort. (I've not had the ability to witness it, but I prefer to think in terms of avoidance...) 😉