Or two gtx580 and ad another one later for 3way SLI.
If those 1k do not include the water kit for the cards it will be ok, if not, 2x gtx570 with water cooling kits will go underneath 1k.
6970 in crossfire(2) will beat a 570 sli(2), but not by much. Since nvidia drivers are more stable or less buggy (by my knowledge) in multiple gpu configurations
, nvidia should be the way to go.