If you bothered to use the forum search function before posting, you would've found this topic has been covered many times in the past.
Many people around this place will recommend the Opteron. The Athlon64 would stand no chance against the Opteron and Xeon. Are you planning on dual processing?
As for control panel, there is no one answer, only personal perceptions. That said, I like CPanel for it's flexibility, functionality, and it's brand name for hosting customers. I like DirectAdmin for it's price. You'll need to make your own mind up
Like most have already said, I personally prefer the Opteron over the Xeon. Best of luck with your choice!
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