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The GXT 590 is 2 GTX 580 cores but downclocked quite a bit.
The Core Clock and Shader Clock are around 20% slower, and the VRAM is around 15% slower, compared to a GTX 580.
So in performance it's a fair bit less than GTX 580's in SLI and closer to GTX 570's in SLI.
It would be a poor upgrade in terms of performance from what you have now.
hmmm so maybe get the 580s but in SLI for around the same price as one 590??
Its not though is it.
1 GTX590 - £575
2 GTX580 - £700
You got a good psu?
Edit, your running GTX470 sli, you must have a good psu.
only got I5 2500K, not clocked as I have no idea how.
Clock your 2500K as that should yield good performance increases over stock and see if that improves things with The Witcher 2.
Good guide to get you started here: http://www.clunk.org.uk/forums/overclocking/39184-p67-sandy-bridge-overclocking-guide-beginners.html