As far as whether you should go with a native DP monitor vs. the adapter, the choice is yours. If you want to spend more money for a more stable technology, then by all means, do it. The thing to note that while a DP monitor seems to rarely flicker, for some people it does happen. So it's possible that it might not fix much.
As far as having 2x21" and a 23", I don't think you're going to like the outcome. All 3 monitors need to be the same resolution, so one being a different size will throw the perspective off, (it's really noticable with a 24" and a 19" at the same time :lol: ). If possible, I would recommend to have all the same size monitors, whether you upgrade all to a larger size, or you keep your current size.
And yes, you can get another card for a 4th extended monitor, but you will not be able to crossfire it. If you crossfire it, you will lose the extra monitors. That being said, if you aren't going to crossfire, since your computer would not benefit from the extra frames, you don't need another 5870. You can get the cheapest video card that supports the monitor inputs you have. There would be no point in getting a matching card if you aren't going to crossfire.
Depending on how many PCI-E buses you have, you can possibly crossfire and have a 3rd card just for the extra monitor outputs. I have 2 5770's and have a incoming GeForce 8400 GS just for extended monitors. But if you don't need the performance gain, there would be no reason for the go with another card like the 5870.
Hope that helps.
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