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Would you recommend having more than 1 monitor? What sort of performance hit do you take from that?
I HIGHLY recommend it. The quality of life improvement makes it extremely worthwhile. Put your chat windows on one monitor while you browse the web on the other, or use SE, etc.
As for performance impact, I haven't benchmarked it, but it seems to be small. The performance I get in games is about what I would expect from my card on a single monitor. The overhead for just having your desktop extended is minimal I think. If you want to game on multiple monitors, that's another story.