To some extent a CPU is a CPU is a CPU. There are AMD CPU's that outperform Intel CPU's and there Intel CPU's that outperform AMD CPU's. As a general rule, dollar for dollar, you get a better value from AMD CPU's, but if money is no object, you get somewhat better performance from Intel CPU's at a higher price. For TSV's price is obviously a factor, so you'll see more AMD processors.
From a gaming standpoint, CPU's typically aren't the bottleneck. The video card is more likely to be slowing the system down than the CPU. You can build a blindingly fast gaming computer using a relatively low-end CPU as long as you've got enough memory, and a blazing fast video card (or more than one.) The video card is where you really want to spend the money on a gaming computer. Better processors are more important in compiling large video files and things of that nature, but as a rule gaming doesn't put excessive demands on the CPU. You can economize a bit on the CPU for a gaming computer and still build a blindingly fast gaming computer. If you economize on the video card, no matter how good the CPU is, you'll have sub-par performance.
HP and Dell (and every other computer "maker") actually uses one of seven or so Chinese/Taiwanese firms to build their products and many also use the same firms to design their products. You can't make any broad generalizations as to which brand is "best" as each brand is made in one of the several factories and your Dell computer may come out of the same plant as a Mac and be made by the same workers. It will likely be designed differently and use different parts, and truth be told that can even change right in the midst of production, so even two computers of the same model from the same brand can vary wildly in quality.
Fly!!! Eagles!!! Fly!!!