Here are my thoughts about the 6xx series
1) In terms of performance, you get less for your money with the 6xx. For example, a 630 costs about the same as a 540, but is usually slower, because the cache isn't as good as the extra 200MHz
2) In terms of features, the 6xx is quite nice. EM64T is a welcome addition, as is the NX bit. I'd feel much more comfortable buying one of these in terms of useful life.
3) Amazingly enough, the 6xx series manages to tame the furnace tendencies of Prescott, despite adding ANOTHER 40 million transistors to the core. Power usage is still higher than 90nm AMD parts of the same grade, but is often lower than the 130nm AMD parts. Considering the ridiculous temps the 5 series was having, this is quite an accomplishment.
4) In terms of value, things change when you talk about overclocking. The lower multiplier and lower heat of the 6xx series might make for better overclocking value. For example, if a 630 hits 4.0GHz, and so does the 540, then the 640 represents the better value, and will likely run a lot cooler, too.
5) Unfortunately, Intel still hasn't combined performance, features, and price together to make a better value than the A64. Either you can have close to the same performance as an A64 (albeit in differing benchmarks) for close to the same cost but with less features and more heat, or you can have all the features and temperatures of the A64, but with lower performance. Until Intel combines both, I still have to recommend the A64 over either the 5xx or 6xx series.
That being said, if I were in the Intel only camp, I'd definitely take the slight performance hit and go for the 6xx series due to the features and power management, and possibly overclocking potential.