Deffo go for the single G80. SLI works best when benchmarking, other than that the real world gaming performance differences are not much, and are some times even worse than a single card. XMS and I both noticed this back on the 7 series cards.
G80 is wicked fast and has full DX10 support, definitely the way to go IMHO.
AMD FX-8370 @ 4.7Ghz | Asus Sabertooth 990FX R2.0 | 32Gb G.Skill Sniper DDR3-1866 | XFX Radeon R9 390X
Samsung 850 EVO 500Gb SSD / OCZ Vertex 4 128Gb SSD / 2Tb WD Black / 3 x 2Tb WD Green
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