[strike]I have no doubt in my mind that a pair of 4870X2s could take out a pair of GTX 260s, especially given how much more mature the drivers will be then.
However, Remember that price/performance is one of the key points the Radoen HD 4000 series constantly hits on, and when it comes to that... The GTX 260s would cost you next to nothing to get due to the step-up program. So seriously, it'd be just cheaper to get a pair of GTX 260s than to shell out what would be like a grand for a pair of 4870X2s.[/strike]
[edit]Whoops, ignore that; I read it originally as a PAIR of 4870X2s... Imanidiot. But yeah, a single 4870X2 would likely beat a GTX 260 SLi setup, since it appears that dual-GPU cards tend to best dual-card setups, when both are based upon comparable GPUs. However, it remains the same... Why pay $500US for what would wind up only being a marginal performance increase over what you could get through step-up for much, much less?