I've always used AMD and have been nothing but pleased with their performance. The big difference between Pentium and AMD is that Pentium chips are already overclocked whereas AMD aren't.

And dual-core are just two processors in one. So basically... a 3GHz dual-core chip is really like having a 6GHz chip... And then there are the quad-core chips....

Ooh and another thing... DO NOT GET A DELL. And I've heard awful things about Windows Vista... I would try to find a comp with XP on it still, if that's even possible. But I've heard that lots of things are still buggy and lots of programs aren't compatible, etc.