Re: AMD Sempron 64
I agree with TRDCorolla. However, if you get a high-end sempron, such as the 3300+ and you pair that with a high end video card, such as the GeForce 6800 or 7800 line, you could probably run most games fairly well. You may not be able to crank the settings to high, but you should get decent performance.
But if you're really into gaming, skip the Sempron and go with the Athlon series.