I would agree with patonb for the most part. If you're going to spend less than $200 on a processor then buy AMD. If you're going to spend more than $200 buy Intel because they are clearly faster.
Don't buy an i5 or an i3 though. You'd be better off getting a Phenom II x4 than an i5. You'd be better off getting an i7 than a Phenom II x6. In terms of pure gaming performance I mean.
If you want super duper multi-tasking power and media encoding strength, then go for a Phenom II x6.
The Phenom II x4 965 will give you roughly the same performance as Intel's i5 750 with the cost of $165 vs $195. The biggest difference is in the cost of the motherboards, though, as AM3 boards tend to be much less expensive for the most part (you can always find stupidly expensive boards if you want them).
The Phenom II x6 1055T is $200, BTW.
Intel's architecture is such that clock for clock it's doing much better than AMD. Which is why Trotter said the i7 750 and Phenom II x4 965 are roughly the same performance. If you overclock then things change dramatically, but you're paying a big premium for Intel.
Don't forget that AMD is coming out with a new chip. They won a 1.8 billion dollar settlement from Intel, and I think it's safe to assume that some of that money went into Research and Development of a new core. I wouldn't count them out yet on the clock for clock performance game, but they are definitely top dogs of the performance vs price segment.