5850s are more powerful, but the 460 seems to really give it a run for it's money in some benchmarks. I think, bang for your buck, going with SLI GTX460s 1GB will be better if you're not gaming over 1920x1080 just because the performance is quite close and you'd be saving roughly $140.
But yeah the CF5850s will technically be performing a bit better under the majority of cases. As for OCing, they both have good OC potential probably rather equal.
+1, exactly what I was trying to say but you said it better.