I second Doc's reply.
The GX2 is probably the best choice for right now.
I do believe the (2) GTX cards in SLI would preform better overall due to having two separate 16x or 8x PCIe slots for bandwidth instead of having two GPU's sharing one PCIe 16x/8x bus. ...but, realistically either option would be overkill for quite some time. With the GX2 card being cheaper than (2) GTX's and getting about the same performance right now - it'd be the better option. Then you could add a second GX2 card later for SLI GX2's if you need to(plus card would be cheaper later).
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Veni, Vidi, Velcro...
Lev. 25:44 states that I may indeed possess slaves, both male and female, provided they are purchased from neighboring nations. A friend of mine claims that this applies to Mexicans, but not Canadians.
...Why can't I own Canadians?