I've built one system for a customer with a Phenom II X2 550 BE. I wasn't particularly happy with the fact that the processor ran significantly hotter (~40C idle) than a Phenom II X4 945, a Phenom II X3 710 or any Athlon II X2 I've used (the rest about 30C idle). In hindsight it seems almost unbelievable that the CPU fan span faster on that processor than any of the others I've used in system builds, yet the CPU temp was higher, but I definitely remember thinking "that's the last time I use that processor". Every time I was using retail CPUs, which all have the same HSF.
Next time I'm with that customer's computer, I'll check the fan speed and temps. Admittedly the fan speed wasn't high enough to be irritating (all the other systems run virtually silent, to the point that I can hear the booting beep echo inside the case).
I agree with koshman that pretty much any 400W PSU will be fine, though I tend to invest in the better ones because they're quieter, and because a failing PSU can sometimes take the board with it, and I believe a better quality PSU is less likely to fail.