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Don't know how this post surfaced for me but..
Sounds like you are on the edge of 1gig memory limit of the vid card.
If you run out of usable memory the system starts up a cache in main mem'. This gets swaped into bid' via PCIe of corurse. If the secound or third slots are not 16x, like 8x or 4x (1 each -yikes!), then you will get a huge dive in performance, 3 16x dives too but not so much.
..on the edge. 1 runs fine, add 2 more is quite a sizable chunk. Most AA modes want a second copy of the frame buffer or more. So 3x the frame buffer useage with AA is 6x screens worth of mem'. -At-least-.
(Isn't there a mode that makes a 4x copy then down samples ,,or is that texture related?)
So, when you were -fast- on 2 you were done. Adding a third would have been even faster. Upping the settings pushed you over 1gig. mem' limit.
..and NO, 3 cards does not give a 3gig frame buffer. Each GPU needs its own.
Eyefinity 6 cards have 2gig mem' for a good reason. One we triplets should have noticed.
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