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Whether or not it would be beneficial would depend on if the games you play most have decent SLI support, if you processor is powerful enough to not bottleneck the cards and also what resolution you play at.
As far as I know the 680 chipsets are dual x16 (assuming 650s are dual x8)so the mobo shouldn't be an issue.
So the mobo and CPU are no issues which is good, but I don't think a second 260 is necessary at that res. It will improve framerates but (in my eyes at least) not enough to justify a second £200+ card.
It's all down to if you're willing to spend a lot of money on not a lot of gain.
Oh yeah, this is also assuming that your PSU is up to the job.
http://www.overclockersclub.com/reviews/xfx_gtx260xxx/8.htm check out this review, it gives a really good idea of the -lack- of performance increase with a second card at that res. compare the SLI 260 in blue to the single 260 in purple.
The conclusion also gives a pretty good run down.