Performance: They're about equal, with the 5830 winning in some titles and losing to the 460 in others. It's the reason Tom has them rated as equal. However, I'm not sure about overclocking potential. I believe the 5830 overclocks better, don't take my word for it.
Heat: Depends on which model you get, according the Hardware Canucks review, for instance, the 5830 they were using was cooler than the 768 mb gtx 460.