Are you going to install 2 vid cards right now or are you waiting to buy the second card later? Do you OC?
Out of curiosity, why are you going SLI? I think the price/performance is horrible and only recommend it if someone meets the following 2 criteria:
1. Money is not a concern.
2. The person games at high resolutions (1600X1200 or higher) and wants max AA/AF enabled.
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<font color=red>You're a boil on the arse of progress - don't make me squeeze you!</font color=red>