I have an X2 3600+ Brisbane (65nm) that runs on 0.975v at full speed. It's Prime stable for 1 round. It never topped 32C, using just a stock AMD cooler at 7V, which BTW was a delta fan that sounded like someone cutting through steel. I have the system in an NSK3300 with a Yate Loon D12SL-12 @ 5v on the rear exhaust, and one Nexus 92mm fan on the bottom intake vent. I'm going to put a Nexus 92mm on the stock AMD cooler, as soon as I can figure out how to do it.
Here's some good clues when you read between the lines. 32C ...stock cooler...undervolted fan,under load. You can find FAR more efficient coolers-and you can do swell letting peak temps go 10-15C higher.....meaning loose more fans.
The 65nm process is more watt efficient and cooler. the "performance hit" is too minor to matter unless you are a bleeding edge game guy. The punch a dual core 3600 packs is way beyond a Barton. In fact-a 3000 Sempron probably performs at least as well as the Barton-run WAY cool
You can transfer most socket A stuff,RAM,AGP to a 754 mobo. 754's are vanishing,but get it soon and it's cool. In 754 the 3000 and up Semprons have Cool+Quiet,the 2800 does not. The re are some Venice 754's...just a few more $-you get more cache,but that adds a tiny bit more heat+wattage.
The TDP numbers are sort of misleading There's a big diff in real heat,real watts between the bottom of the line and the top of the line. In a line,the chip rated as a 3600 and the chip rated as 4800 may have the same TDP watts-on paper,yet the reality is the hotter/faster chip never draws that-and the slower/cheaper chip draws way less.
I'm a bit amused at the hype that Core 2's are real cool runners. In real life it does not tend to be so. You can get a cooler quieter rig with the low end X2...and get even cooler/quieter with Semprons.