Well, in terms of gaming performance I don't really know (the games I've played on Mac work great, but I've never compared a game running on Mac to the same one on PC,) but I can tell you that graphics performance in general is excellent on Macs. Indeed, that is the main hardware advantage of Macs over PCs. They have from the get-go been designed for working with high-end graphics and are the standard choice for computer artists.
Raw clock speed numbers are misleading because of differences in the underlying chip technology. A given clock speed on a Mac is about doubled for the equivalent on PC; thus, the highest PowerPC speeds are about equivalent to the highest Pentium speeds.
Also, a large part of the higher price of Macs is that Apple includes all kinds of pricey gadgetry standard (stuff that is optional on PCs.) This is of course still a downside if you don't need all that stuff, but knowing this tendency of theirs fills in a lot of the price differential.
Raw clock speed numbers are misleading because of differences in the underlying chip technology. A given clock speed on a Mac is about doubled for the equivalent on PC; thus, the highest PowerPC speeds are about equivalent to the highest Pentium speeds.
Also, a large part of the higher price of Macs is that Apple includes all kinds of pricey gadgetry standard (stuff that is optional on PCs.) This is of course still a downside if you don't need all that stuff, but knowing this tendency of theirs fills in a lot of the price differential.