i would like to know the best CPU for my computer ?

8350rocks

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Looks like the highest you can go without adding a new motherboard is an Athlon 64 X2 5600+

Your motherboard is only 95W, so you can't install anything over a 95W CPU. If you want to upgrade further, you could build a new machine. The Athlon you'll be hunting for will be harder to find and likely a bit more expensive, because it's not a currently produced model. Your Motherboard socket is AM2+. The newest motherboards are AM3+.