I have a computer and I want to upgrade the CPU. The motherboard inside the computer supports AM2/AM2+ CPUs. I was recently building a computer with a socket AM3 Athlon II for a friend. Before we INSTALLED the CPU in the new computer, we first tested it in the AM2 board to see if it would be any faster. I ran a CPU benchmark and it was slower. The older CPU was an Athlon 64 X2 5600+ 2.9GHz and the new one was an Athlon II X2 3.0GHz, so I was expecting it to be faster.

Here are my questions:

1. If the CPU is not on the motherboard's support list, but works in the board anyway would there be any reason for it to function slower? Is there some sort of default CPU driver that gets loaded when the CPU is not recognized causing it to run slower? I figured it would work or not at all.

2. How much does an AM2+ socket board limit the performance of an AM3 CPU? Is it better to get an AM3 CPU or the fastest AM2?

3. Do you have to reinstall windows in order for it to recognize the CPU and run it faster?You're lucky you didn't fry that CPU...1) There's no reason the chip should run slower, unless the benchmark is dependent on other things - perhaps it's running the RAM at a slower speed than the older chip, for example.
2) It doesn't REALLY limit the performance, no, just adds/changes certain features.  As for the second question, that depends on many things - budget, other hardware components, what's available, etc.
3. No.