if you wanted to stay with ati your upgrade path would be 6850 then 6870 then 5850 then 5870 then 5970 , in that order, but the cpu you have miught bottleneck you beyond 5850.
Like i said the 4870 is a damn good card though should be running almso tnaythign you throw at it in high settings and good fps
on the nvidia front with sli being an option yu're looking at gtx 260, 270 or 280 sli'd, 460 786 mb , 285, 460 1gig , 465, 470, 295, 480 , or 580 in that order. on the nvidia front after the 460 1 gig you'd be cpu bottle necked
*added*
give us a budget and your psu info and we can make better suggestions, liek the 5970 or 480/580 woudl be great but only if you wanna spend 400-600 on a card