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i have the barebones stock card with 1 dvi input and 1 mini input and a 20 inch lcd monitor
Here's the specs listed by Apple for the graphics card:
NVIDIA GeForce GT 120 with 512MB of GDDR3 memory, PCI Express 2.0, one Mini DisplayPort, and one dual-link DVI port

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.

Or just buy the Mini DisplayPort-DVI adapter for five bucks more and you can use both ports. Either way works well.
 
Or just buy the Mini DisplayPort-DVI adapter for five bucks more and you can use both ports. Either way works well.
😱 But it costs MORE! 😀 😛

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...) 😉
 
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