If you are not bothered about going down the sli route again, then i would go for the ATi x1900xtx range of cards, as these are the fastest available at the mo. A.N. Other mag has the HIS radeon x1900xtx (£358 or there abouts) as their top card. They state ' this is the card to use if you want to get the most from your high-res monitor'.
If you prefer geforce then I would choose one of XFXs pre-oc'ed range (scans 7800 page click here=), though there is now the 7900 to consider you do not get a massive hike in power/vfm over the 7800 series.
Hope this helps.