Personally I would go for a powerful single card.
SLI has its uses.....but as far as I am concerned that is only when you want to take graphics performance beyond the range of the current top single card.
The pitfalls of using SLI with two slower,mid range (or even lower) cards is that not all games are SLI supported.
If you buy one of those games you will be stuck with inadequate performance at the level of a single graphics card.
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