um please research this, but if i remember correctly the 9800 gtx was a less heat producing 8800 gtx , while the 260 was just a slightly upgrade 8800gtx with the less heat tech from the 9 series.
So in therory i wouldnt waste youre money id buy another 8800 gtx to sli or id go for a 285
as youre basicly throwing money into a slight upgrade and for the price the slight 10% increase and 20% less heat produce ant worth the hundreds
1) necro-thread, check the date before you reply
2) neither the GTX260 or GTX280 would be worth it at this point
3) the GTX260 would be quite a bit of an improvement over the 8800GTX as it is a completely different architecture (G200 vs G80)