I'm just another owner, not AMD personell, but I might offer some guidance. I own an X2 4800, X2 5200, and X2 6000. With the subsequent frequency jumps made in these processors, the heat given off increases. If you're applicaiton/processes use a high percentage of the cpu for long periods of time I'd recommend a better heatsink for your 6400, than came with the 4200. I use the Artic Cooler freezer (about $35), and run an app that runs both cores at 100% continuously. Current temp on the AMD64 X2 6000's CPU is 48C and ambient room temp (measured at wall 6" from computer) is 80.4 F (26.9C). I just got home and haven't turned down the AC yet, but the cpu temps are normally about 44 to 45C.
When I ran the stock heatsink on the X2 6000 machine at full load, it ran 61C.
[edit]Dang the price dropped to $20
http://www.newegg.com/Product/...x?Item=N82E16835185125 I also don't work for newegg