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Apparently someone is doing the higher spec GTX460 1GB for £167 if you can find that it would be worth stretching to.
What resolution do you currently use, and what games do you play?
and how much faster is the 460 over the 4890What brand is it?
Will power it up no probs.
http://www.overclockers.co.uk/showproduct.php?prodid=GX-064-GI
Get this one IMO, will save you some cash in the short run.
Yes, it's only the gf100 gpu's (gtx470/480) that don't have hd audio over hdmi.