Its cheaper (on short) to SLI it.
But wait for next gen (8xx/9xxx) and buy that high end card. Reason is that once you get power shortage on 2 SLIed cards you cant do anything but buy better card (or 2 better cards if you will). But if you buy one better card and when this ones too weak SLI it you only have to spend for stronger two but not weaker two too.
Hope you understand& it helps