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Generally it is better to buy one more powerful card than go with two lesser cards. That allows you to upgrade to another one down the road if you desire when your performance is no longer there, and the same card will have dropped significantly in price. Plus initially you avoid all the downsides to multi-gpu systems.
Most of the time you're paying 100% more for 50% or less performance for the 2nd card. There are times when cards are at sweet spots, which allows two lower cards that are cheaper and perform better than a higher priced single card in a lot of games. Again though you can encounter things like microstutter, high power reqs, driver issues and even games that hardly or don't even support sli/cf at all to name a few.
yeah it does depend greatly on what your after and your budget.
For this setup it represented unbeatable performance for the money (when the GTX275 was only $270AUS)
I'd say you would average around 70-80% extra performance in most scenarios.
Have to make sure you have a good case with plenty of cooling.
right now the only SLI setup that's worth in my opinion is a couple of GTX260's...but even they have gone up in price...try ebay.